There's a massive difference between evil, and jerk. Evil characters don't have to be snarky moustache twirling villains. They are people too, they can have friends, and laugh, and have fun. Crazy I know. Adding conflicting personality traits to characters is what makes them feel more real and relatable.
Most of the companions act like they are not 1st level characters. They most likely accomplished more in their adventuring career hanging with the PC than ever before. Laezel is acting like killing a camp full of goblins is just another day at work. Nothing special. She most likely haven't done anything close to it in her life, and still every time I approach her I get this "SPEAK! QUICKLY!" like I am working for her or something. The same goes for the rest, they all act like they are some accomplished adventurers, their stupid attitude is not reflecting their actual in-game power.
Lae'zel is a proper level 1, she's still seen as a child per other Githyanki. She's just full of herself and think she is better than every other races as all Githyanki do.
Astarion wasn't a playable class before the intro events, just a socialite that brought diner to his master the last 200 years. The tadpole physically affected his condition.
Wyll is probably lying about his feats, considering the goblins think he is a joke and aren't scared of him.
Gale pissed off a god who used level drain on his ass.
As for Shadowheart,
she gets periodically mind-wiped
