Yeah I believe if you want to make it believable you need to rationalize the "bad" behavior or idea's, you want to give good reason behind the thought process... like maybe telling a party member "It's okay to do X bad thing, because the person they are doing it to deserves it" or "It's fine to kill these people cause they're beyond saving, there's no other way, if we don't kill them now, other innocent people will die", etc.. It really depends on the situation, but you have to make it seem like the main character is rationalizing, and normalizing the "bad" deeds of their fellow party members, it makes it more believable to put a believable emotion behind this.
Like maybe your party member's family was killed by the general of X army, and when that party member encounters said general later down the road, they must make a choice to either kill, or forgive the general, who's may or may not have changed his way since then.
In this situation the main character can provoke the party member by telling them to think about all the suffering he's caused, and all the other people he's done the same thing to, think about all the lives he's taken, and how he deserves to die. or something along those lines. and to make the situation more relevant you can add a flashback of what happened, and how terrible a person the general was in that situation, so you can attempt to make the player feel some degree of malice toward the general to make them more inclined to choose to kill the general.
This could then start the party member down a darker path, because maybe later he meets someone somewhat bad, and decides, "he could turn out to be just like that general", so now he has an excuse to continue killing people he dislikes/disagree's with.
Sorry for the dark take here o_o; but this is just an example, it's best to think about each characters personality and traits individually, and think about where their weakness lies, these are things can may be exploited by the main character to begin turning them "bad".
Hopefully this is more helpful ^^ I'm not writer myself, my skills lie in programming, but this is just my take on making the situation more believable. Maybe someone with more insight may be able to provide better feedback on this ^^;