I've noticed many of my players' characters have certain qualities that clash with their morality, and I like this fact and want to build off it. Each of my players has something like this, and I'm wondering how to exploit these qualities to somewhat influence a change in the characters' morality. To be clear, my players are fine with this, being interested in the roleplaying aspect.

The Rogue in the party has been exploited by their guild and has been between being a moral person and a bloodthirsty murderer for a while. I want to drive them over the edge to some extent. Their character's main arc is trying to balance the good and the bad, and I think having them briefly relapse before fully upstanding would be interesting.

The Fighter has been trying to keep themselves together for a while now, but they're likely close to a breaking point after revisiting their home only to find it invaded. The most obvious route is that they go on a full path of vengeance, but then slowly go down a darker route through the mindset of "sins of the father" in which they harm people who weren't even involved but were associated with the person.

The Barbarian is also a reborn, and lost all their memories after their resurrection. While they now have been told by the party (who they knew before their death) that they were royalty, it hasn't set in just yet. The idea is to have them go mad in an attempt to ascend to the throne that was their birthright but lost because of their death.

The Monk is a different story. After they took a cursed blade off the hands of a dying comrade, it slowly corrupted them, twisting their values to be darker. Above all else, they value their allies, wishing to keep them safe and out of harm. Considering the blade also gives them a thirst for violence, I think a form of selfishness and pride, believing the others are weaker and insignificant, not deserving the honor and spoils of combat would work.

These aren't set in stone, and none of these have been fully realized just yet so I'm looking for ideas. Any suggestions?

What is the goal though? If they want excuses to be murder-hobos or you want to make them be murder-hobos sure those all work. Though personally, I find it weird that you are planning out the personalities of your players' characters. DMs are mainly supposed to present situations and allow the players to choose what their characters would or wouldn't do.

Quote from Agilemind >>

What is the goal though? If they want excuses to be murder-hobos or you want to make them be murder-hobos sure those all work. Though personally, I find it weird that you are planning out the personalities of your players' characters. DMs are mainly supposed to present situations and allow the players to choose what their characters would or wouldn't do.

 The goal is mostly narrative. The campaign in question's main theme has been good and evil, and the grey in between. The players on multiple occasions have been kind of treading the line between them. And you're right about the "presenting situations" thing. My main goal is to present situations that can present this change.

Quote from Fr0st_FirE >>

Quote from Agilemind >>

What is the goal though? If they want excuses to be murder-hobos or you want to make them be murder-hobos sure those all work. Though personally, I find it weird that you are planning out the personalities of your players' characters. DMs are mainly supposed to present situations and allow the players to choose what their characters would or wouldn't do.

 The goal is mostly narrative. The campaign in question's main theme has been good and evil, and the grey in between. The players on multiple occasions have been kind of treading the line between them. And you're right about the "presenting situations" thing. My main goal is to present situations that can present this change.

 Sorry I still don't really understand what you mean by "present this change". If you want to explore the morality of the characters, give them situations of unclear morality and allow them to choose. Even better, set them up in situations where the morality isn't what they expected it to be going into it. 

For example, I had a great morally complex situation for one of my vengeance fueled characters where they found out an NPC was selling fantasy drugs to children, but when they went to the NPCs house they found them living with their disabled mother with a stack of unpaid bills from the herbalist for their mother's medicine. Or in another situation I had the party track down a hired murderer while the murderer was visiting their wife and children.

Another good one I had was the BBEG recruiting a family member of one of my PCs to their cause - I've done that a few times with different players and it usually results in a good moral debate about the merits of what the BBEG is doing. Alternatively, having multiple antagonists that oppose each other trying to convince the party to take out their opponents is a good one too. I had a crimelord vs a mind-controlling Aboleth fighting over control of a city in one campaign, and a coven of hags vs a dragon in another.

Specific suggestions:

"The idea is to have them go mad in an attempt to ascend to the throne that was their birthright but lost because of their death."

• the current king could be a good ruler if a little harsh on some topics, maybe they have outlawed all religions but have started a technological/economic renaissance.
• even if the current king is a bad ruler, some of the people working for them might still be good and trying to help the city by trying to avoid civil-war.
• have a devil offer a rumplestilken-type deal where he can get his throne but has to give up that which he loves the most, then have it so they must kill their mother or sibling in order to ascend the throne and if they refuse the devil does the deed for them.
• Or have the holy lawful-good church oppose their ascension to the throne because they are an undead creature - if they attack the church in response show all the good they do - e.g. have part of the church filled with bedrooms for orphans, show the priests/priestesses handing out sack of grain too the poor etc....

but then slowly go down a darker route through the mindset of "sins of the father" in which they harm people who weren't even involved but were associated with the person.

• Perfect opportunity to have the culprit have a wife/partner & family who will swear vengeance on the Fighter if the Fighter kills the culprit.
• Alternatively, have some young misguided teenager(s) be part of the gang who invaded, or even have one of the Fighter's younger siblings working for the invaders.

I want to drive them over the edge to some extent.

• Have them meet an NPC just like themselves working for a different guild that is exploiting them but have that NPC stay loyal to the guild and try to kill the Rogue for the guild should the Rogue try to interfere with that guild.
• Alternatively, have the guild start exploiting someone the Rogue cares about to the point that the guild gets that person killed. e.g. younger sibling joins the guild b/c big brother/sister is in it and the guild gets them killed.

Above all else, they value their allies, wishing to keep them safe and out of harm.

Easy peasy if your group is as criminally minded as it seems. Just have a legitimate honest guard start investigating one or more members of the party for some crime/massacres they did and have the curse weapon tell the monk the easy solution is to kill the guard. 

Thanks! All of this helped me with the ideas. When I say "present this change," I mean to make a situation where the choice can be made so the party can start the descent. I just needed a plot point to hook the players.

Quote from Fr0st_FirE >>

Thanks! All of this helped me with the ideas. When I say "present this change," I mean to make a situation where the choice can be made so the party can start the descent. I just needed a plot point to hook the players.

Keep in mind a player may never go down the path you want. And if they do, then they may never go for redemption.

It also seems you are heading towards an evil campaign and if that is what you want, then all is good. You just have to be careful. 

^^ Very much that. You cannot and should not ever know which choice the players will make when you give them a moral quandry. You should always be prepared for them to take either path. Likewise, if you drive the PCs to be evil don't expect they will ever return to being good, be prepared for them to side with your BBEG and become tyrant overlords of your world. You should also be prepared for intra-party conflict, IME it's harder to justify a bunch of evil characters working together than a bunch of good characters working together. 

Though TBH, I find if you really show the players the evil of the evil choices most of them won't choose to do it. It's one thing to say they want to explore being evil when just thinking about killing NPCs that annoy them or having an excuse for over-the-top violence against an antagonist. It is quite another to RP killing a NPC in cold blood in front of their 7 year old daughter, or burning down a church filled with orphans, or killing one of their own family members.

Just to reiterate, give them opportunities rather than courses of action. What they do is always ultimately up to the player, and if there's anything we know about players, it's that they'll often surprise DMs with their choices (for better or worse).