Dia could probably like, kill someone very easily with her mind

Dia could but she wouldn't because she is the atypical good girl of the group and therefore baring something really bad happening will shy away from the bad stuff that is what Arntte is for

Tych casually callin' Arntta a bad girl

wow

Dis's got a kind heart. She'd have to be backstabbed very very badly for her to go evil on that ability.