both because chara thinks he's too rash and throws himself carelessly at other peoples problems and gets hurt as a result and also because they think the irony is funny
Deku has no idea Chara still thinks of him as a friend I think. Hes aware they have strong goals in common, but lord does he ever expect to be stabbed again while trying to figure that out
they would be less likely to do it when they're not in july murder mode but it's a conscious choice to not stab when people intentionally get in their way