I'm going to key his car
Why key his car? Is he a jerk?
Yes, he's a jerk and a cheater
I know you're upset, but it's wrong to damage someone else's property. You need to take a calmer approach.
Don't care, I'm going to key his car just because he slept with my sister's best friend.
That must've been devastating for her but it's no way to solve anything. You could be sued or even arrested for damaging his property. They need to work this out themselves.