It's wrong to write things on other people's cars.
Colin pissed off Jill so much that they ended up breaking up.
Jill wants revenge against Colin.
Jill spits on Colin's car without him knowing and then leaves.
Jill wrote a Nazi symbol on Colin's car using a car key.
The spit is barely noticeable so Colin is unaffected by the act.
Colin has to ride around with people staring at his car, and he has to wait to get it off.