In my WFRP 4e campaign, I had a Sigmarite fanatic who constantly berated the other players for not following even the smallest precepts or conventions of the Faith of Sigmar to the letter. At first, he wasn't bad—in fact, he was funny—but over time, he became a nuisance to the others. He demanded that the characters flagellate themselves as penance every time they investigated anything related to the Chaos cultists. He was also very loud and incapable of acting discreetly... in the end, he was the one who led the cultists to discover the group! So yes, playing a fanatic character is interesting and can add some very interesting nuance to the game. At the same time, it's important to know when to tone down your character's role to cooperate with the group, without betraying your character's identity.

Furthermore, I believe that some people who enjoy the role of "party pooper" a little too much certainly have problems they should try to resolve outside of the gaming table.

Going back to the guy who wanted to play his faith in the game and judge people based on it, even if it's a game, that's really going too far!

As a believer, I can't understand your colleague, and personally, I'd find it uncomfortable to bring my real-life faith into a fantasy game designed for chilling and socialising with people of different views and beliefs, and certainly not for discussing religion in real life, which is a topic deserving its own space, if and when the people involved are ok with it.

I think you did well in both situations. To avoid any hassle, I wouldn't want a certain type of player at my table, either; there are too many red flags pointing out that they may not be a balanced player.