So long-story short, my party has stumbled into a massive conspiracy involving the ruling monarchy and their rival guild. During their travels for their own guild, they found themselves in the middle of a massive terrorist attack on a city. The perpetrators were disguised as members of the party's guild in order to frame them, and now they have been arrested for their involvement (one party member even crit failed an insight check on the perpetrators and started helping them slaughter civilians). This new session, they are going to be put on trial by the king and I'm trying to figure out how to make this still be fun for my players. Has anyone ran a session similar to this?

Not one where any of the players were, in fact, guilty of mass murder.  There might be a DM lesson there on the consequences of crit-fails and why it's usually just easier to say a one is a normal miss.  Because this isn't even a frame-up and they aren't even being railroaded.  The party actually participated in this.  The trial isn't going to be fun because the conclusion is not only predetermined, it's correct.  

Say, for example, someone staged a coup attempt wearing very obvious paraphernalia associated with a guild or political faction (flags, hats, whatever) and a bunch of people got killed and then the rest of that faction all just stood up and said "Nope.  Wasn't us.  All those people hanging around in the taverns and the town square bragging publicly about doing it and  about their participation?  They're all faking it and are on the other team.  All the fliers we printed up inviting people?  That wasn't us either.  Someone stole our printing press and faked all those."   

Well, any jury in the world would laugh in their faces, wouldn't they?  That anyone would be such a loathsome unprincipled coward, brave enough to kill people while they're in a mob and crying about their honorable intentions afterward?  You couldn't even call them adults anymore.  It's what a five-year-old does.  It's disgusting.  I'm afraid those people would have squandered their credibility with anyone besides their fellow-travelers forever.

So, that's your basic problem.  Not only does it look like your players' guild did it, your players, representing the guild, actually did it.  Now, if the corrupt king were on your side, you guys could probably expect a corrupt-but-legally-effective pardon.  After which, the king's political faction would make sure you landed on your feet.  However, since it's in the corrupt king's interest to bury your players AND they actually participated in mass slaughter, I'm afraid he's going to win this round.  

You're probably better off hand-waving past the trial and skipping right to the jailbreak.  Or else your players can try to switch sides and offer the corrupt king false testimony against higher-ups in their own guild.  Then the corrupt king could further his political ambitions by having his corrupt judges announce a sham investigation into his political enemies.

Okay, here's what you do:  the corrupt king sends a crony of his to the prison to see if the party would be willing to save their own asses this way and when the crony leaves, he leaves behind a sheaf of papers to be signed, held together with a paper clip that the rogue can fashion into a crude lockpick.  That way, they can break out with their honor intact, or such honor as they have left after killing a bunch of civilians without even knowing why.

The thing about those royal trials is they don’t need to be, and often are not, fair. If the king is holding one, it could be because he is just and wise and wants to find the truth. It could just as easily be that he has already decided and is holding a show trial to give his actions a veneer of respectability. So the first question would be what kind of king is this, in the case of this trial?

Start there, I’d say, and it can get you a long way toward how it will resolve in game. 

Quote from TimCurtin >>

Say, for example, someone staged a coup attempt wearing very obvious paraphernalia associated with a guild or political faction (flags, hats, whatever) and a bunch of people got killed and then the rest of that faction all just stood up and said "Nope.  Wasn't us.  All those people hanging around in the taverns and the town square bragging publicly about doing it and  about their participation?  They're all faking it and are on the other team.  All the fliers we printed up inviting people?  That wasn't us either.  Someone stole our printing press and faked all those."   

 Hm, why does this sound so familiar?

I'm sure I have no idea what you mean.

This honestly does sound familiar.  A thread from long ago.

If the characters run afoul of the legal system, magic should be able to sort it out.  A Zone Of Truth spell is very helpful.  Some Speak With Dead may help.  Commune perhaps.  If all else fails, Charm spells or Telepathy may do the trick.  Such things are much better than Torture or Trial By Combat.

In a fantasy game in which magic works, raising your hand and swearing by the gods to tell the truth, the whole truth, and nothing but the Truth is quite literal inside a Zone Of Truth.  The gods may really be watching.

Given that your king is in the conspiracy and oviously biased against your character, its best to make a quick mock trial, and then, like a previous poster said, skip to the jailbreak. This isn't a fair trial, this is being guilty AND up against someone who obviously wants you gone.

Real trials are boring as all heck unless you have a real passion for the nuances and technicalities of legal argumentation; and even real lawyers prefer not to litigate unless it's absolutely necessary (for a lot of reason not just because it's boring).  

If the trial is adjudicated by the King, and the party are members of the scapegoat guild, the trial is pretty much a sham to scapegoat the party.  If the justice system in the game allows an actual jury that is fairly picked (and not engineered by the King), then the players have a chance at exoneration.  Do they have lawyers?  In which case the party is in the back seat and it's probably not going to be fun to "play through" sitting at the table while they're lawyer works their butt off or phones it in.  If they're representing themselves, you can have the party come up with a means to defend themselves legally and probably do a mix of investigation (gathering evidence), persuasion (preparing cooperating witnesses), insight (figuring out most effective way to present argument to the jury), persuasion or performance (the "law" stuff you see on TV that's actually not reflective of actual court room proceedings).  

If it's a rail road, and the party is still able to present a defense, they still don't win but maybe their account reaches the public and that may get segments of the public to aid in an escape if not actual uprising to take down the king.  

EDIT: as far as making trials cool: Few Good Men, Judgment at Nuremberg, The Andersonville Trial, check those out and if those are not stimulating, you probably just want to do the trial as brief exposition or a set of skill checks.  Oh, the Finale to The Prisoner is a sort of an absurd trial if you want to go weird.

There are many ways to get out if this but this does highlight a serious misunderstanding of rolling a 1 and how insight checks work. 

First of all a player rolling a 1 would not make him suddenly go against alignment regardless if what he thinks, in addition you should have allowed multiple rolls, he thought they where his friends, then they start killing people, roll again, they ask him to kill people, roll again, every time he us out in a position to question then a new roll by him or someone wise us allowed. 

It might have seemed cool at the table but there need to be real consequences for the character the best of which may be a change of alignment. 

I appreciate what Scarloc is asserting, but I want to say there's also a spectrum from the Call of Duty "No Russian" game sequence and the ultra violence that's frankly just shenanigans tonally in something like a Lobo comic or a Borderlands/Duke Nuke'em type game.  The "seriousness" of the crime in your game world should determine the tone of your trial.  Maybe it is a grave matter (arguable moral realism) or maybe it's further absurdity and itself ends ultraviolently.  Like do characters in Grand Theft Auto ever really face accountability?  If that's more tonally accurate the trial is just another platform to jump off of into the next mission.