I wouldn't do anything besides tell my PC to "roleplay it out". Depending on the way they went with it, I might start having thing happen that lined up with the story they wanted to tell, if any.
What would you have your players do if a "good" character wants to attune to an evil sentient magic weapon? make a roll to control? not be able to use its magical properties? etc. please advise.
If the weapon is smart enough, it may play along at first in order to get the PC to rely on it before making demands/request.
If the weapon is patient enough, it may try to corrupt the wielder slowly over time, like a little cartoon devil sitting on her shoulder.
If the weapon is sadistic enough, it may let the PC rely on its abilities then withhold them at a crucial moment and watch with glee as the PC fights for survival against significant odds.
If the weapon gets off on tearing people down, it may relay the PC's secrets to other PCs (maybe it shows the other PC in a dream what the wielder PC has secretly done).
Or, the weapon could be so overtly evil that it just won't function for you, or that it hurts you to use it.
