Game Dev Here

I'm making a FPS shooter game and I'm wondering what type of mechanics are seen as bad in terms of character building

I wanted to have skill trees, the game play is kinda like Dead By Daylight, Deceit but you have to kill the monster or the monster has to kill you.

There's no bad mechanic per se, but pitfalls are everywhere. Skill tree is actually one of the hardest way to build character because of the balancing issue. Remember that EVERY character with EVERY skill branch must be viable else most wouldn't pick them up at all. I suggest for your first game you should just drop the whole skill tree thing altogether and focus on the gun mechanic only. You may think a conventional shooter is lame but whatever you produce will be 10 times better than the Dead By Daylight clone you're making.

Thank you for the response, I apperciate it, also wish people who down vote would say why instead of just randomly down voting.

I will focus on the gun mechanics a bit more, thanks for the feedback again : )