I think you have a great tabletop character idea but a pretty problematic larp character, (religion aspect aside for Alliance in particular). Can it be done? Sure, but it would require a tremendous amount of effort on your part to make sure everyone is having a good time. Remember, you are not just here for your own fun, we are here for each other. Your idea of fun may not be theirs and this idea in particular is an intrusive one that will impact others whether they like it or not. Like others have said, NPCing would be the best option for this concept.
Playing an “evil” character that is prone to killing others is fun so long as the whole group is in on it. In a tabletop game, you can discuss your concept with the other members of the table. You can find out what would be fun for the PCs and work out ideas with your DM.
This is not so true in most larps. You will not be able to check in with every single person that you play with before you create your character. Larp players come and go. New people and visitors will join your game with no knowledge of you and they may not find what you are doing to them very fun at all. In most other cases, they could simply not associate with someone they didn’t enjoy playing with, but if you are directly preaching and threatening to kill them if they don’t believe your object of worship exists, then it’s not something they can just ignore.
This would be fine if Alliance was designed as a pvp or faction based larp, but those are just possibilities, not necessarily the point of the game. Also remember that this game does have a perma death mechanic. After dying a certain number of times and eventually running out of luck, a PC can be permed. So most of the IRL money and time they spent on their characters, hundreds if not thousands of dollars for most people, is gone as most people wouldn’t want their next character to wear the same things.
This is the main reason my LARP characters are vastly different from any of my tabletop characters. My tabletop characters are usually prone to risky behaviors that are fun and humorous, but often bring the party much closer to risking death. When you take actions that greatly increase the risk of death for an Alliance PC, you are not just risking their character’s life, but the IRL investment of their character. It would be fine if you talked about it OOC with the PC, but the chances of everyone at an event agreeing to it would be extremely small.
There was actually a discussion about antagonistic characters on /r/larp a few months back. Someone pointed out that these types of larp pcs usually operate by taking advantage of unspoken agreements between players to not ruin the game for each other. Basically, they are manipulating others into acting a certain way because they know most folks don’t want to be seen as a player that is stopping someone else from playing the way they want to, i.e. killing them or rallying other PCs against that antagonistic player. Another person in the thread mentioned that there are meta-expectations and social rules attached to players that are not attached to NPCs. Most of the ways with dealing with antagonistic PCs will not be fun for that antagonistic PC so folks are less likely to commit to doing it. If it were an NPCs, they wouldn’t hesitate.
My advice for anyone wishing to play an antagonistic character in a mostly PVE-centric game would be to find another PC and talk to them about how they might enjoy interacting with you. You don’t have to stage every single interaction, but just give each other a general sense of what your expectations are and where you draw the line. It will not be completely organic, but it will ensure that you aren’t ruining the experience for others.