I'm new to D&D and so are the friends I play it with.
My character is a Chaotic Evil Pact of the Tome Warlock. One of my party members is a Lawful Neutral Warrior, and in his backstory he dislikes mages because of what happened to him/his family. In total, there are four players
Whenever I use a spell in combat and he can use his mage slayer feat, he always does. (This situation stemmed from me hitting him when I critically failed). An example of this was today when I was fighting two fairly powerful monsters and taking a beating. I killed one, took an attack from the other, used my Hellish Rebuke spell to attack the monsters and then he attacked me, knocking me out for the rest of the long fight.
We haven't come across any mages that were evil or that we should have fought, so he has only been able to use his feat on me. His reason for KOing me in the fight I mentioned earlier was because I critically failed in a different fight and killed an unconscious monster that could have given our party important information.
This is largely an in character problem, although I did get a little annoyed when he knocked my character out. The other players thought we were a bit stupid to be fighting, but we sometimes mess around in character, putting each other in danger, so that was about it.