Hey guys, so I'm about to DM a game with a few new players and a few veteran players. So I wanted to use a classic icebreaker by opening in a tavern. However I also want to give some narrative hooks for my players. The basic plot of the campaign is that a cult is terrorizing a village and also kidnapping adventurers that are in the local area. Nobody knows why this is happening and so the villagers need a group of adventurers *a group of adventurers is unheard of in my campaign world* to stop this cult. As it turns out the cult is summoning a demon and the cult needs the power and vigor of adventurers to summon the demon. The cult plans to harvest the group of PC's along with many other adventurers. But through the power of teamwork the PC's will be able to stop the cult and save the other adventurers. As it turns out the cult was actually palnning on the PC's working together as it is more effective if the cult harvests adventurers in groups. The PC's will have to fight off the cult and save the day. That's the basic plot of this campaign *I forgot to mention some villagers will also be revealed to be cultists* so I need a great opener to establish all of this while still being mysterious. So I will open with the PC's in the tavern. The PC's will be surrounded by other adventurers as a storm has forced all of the adventurers in the area inside. Anyways, so I will get the PC's to mingle a little bit and get them comfortable. Once that happens I will bring in a dark and mysterious figure. This figure will command the adventurers in the tavern to rise. This is where I will establish that some villagers and adventurers are cultists. Anyways so the cultists will fight the other none cultist adventurers and villagers to the death *the cultists are OP and will surely decimate the others*. However, the cultists will be commanded by this dark figure to instead not kill the PC's. I feel as though this campaign opening will not only establish these cultists as mysterious but also as a force to be reckoned with. Btw I've spoken with the players and they will be creating proactive characters these characters should have ample motivation to find the cultists. They will not only have motivation as character's but also as players because in my experience players don't usually like being treated as weaklings and will attempt to kill these cultists. Note that the entirety of the story and plot of the campaign does not rely on not only the cult but also the PC's motivation to destroy the cult and that the cult is only a small piece to the puzzle of this story and world.

1. What do you want the PCs to do while the other adventures are killed?

2. Why are the PCs saved when others are killed?

I worry that the PCs will say “I just met the other players and they are as important to me as the other adventures getting killed. Let me get involved”

From point 2 I worry that the players would feel like “we where saved only because we’re PCs”  which means it’s a bad plot device that needs some fleshing out.

Perhaps there can be an underground resistance to the cultist that tries to save as many adventurers as possible and guide the PCs and some others out of the tavern and into a safe house?

I think an overall problem is that you want to force a conclusion of the scene you are presenting for the PCs. That can work, but it's in my experience better done if less obvious.

How about:

Let the PCs have a night of fun with the rest of the adventurers in an inn. I would suggest having one or two of the NPCs connect to the PCs. An old comrade in arms, someone they did business with or grew up with. Serve them a bond or two on a platter. Next day, everyone says goodbye and go their separate ways.

Midday a runner cathes up with the PCs. A junior nobody the had no connection to but noticed as a member of the other group(s). She/he talks about an ambush, about cloaked figures and that she/he didn't know what to do, except run after the PCs.

Next up, whether or not the PCs continue or turn back, have cultists show up searching for them and ready to attack. After, it's pretty damn certain that the PCs know the cultists are after them. If you want, add a handout with descriptions of the PCs and that they are to be brought in alive.
However, I think the story also needs something that make the PCs, and the other adventurers, so special. How does "the cult needs the power and vigor of adventurers to summon the demon" manifest itself? Is it a shared experience (they have all fought demons and affected in a certain way), are they marked somehow (by gods, fate, other) or something else? Or is it that the cult does not know what they are after either, just that its a bunch of mercenary types, of which a few are the special victims needed?

One way forward is that this particular story arch ties in to one PCs background. The he/she is special in this case and the required sacrifice. You can get to the other PCs in other adventures and have them be special. This is the time for just one to stand out a bit (but of course need the rest to survive).

I see nothing wrong with this, except that opening in a tavern is the most awkward icebreaker there is. Trying to escape from captivity together is also a very poor beginning.

I've found that throwing the party into adversity very early, like a plague of monsters hitting the village just after they meet for the first time, is a good way of building cohesion among a group of players who don't really know one another (as opposed to starting just in front of a dungeon door, for example). Problem players will try to do things like fleeing the the danger, take all the loot for themselves, and so on, and then say something like"It's what my character would do". You can then purge those players from the table, safe in the knowledge that they will not get better -- only less bad, at best.