He's thinking "I went to jail, so should you". But Goetz said to one of his assailants who he had shot "You don't look so bad" and shot him again. That's probably why he was convicted.
The moral is, finish him off only if there are no witnesses.
Goetz was acquitted of all charges except illegal gun possession. The jurors probably wished he killed all four of those losers instead of just wounding them.