We splattered him with used motor oil and he sat outside for a week.
You are joking about this, right? Why did you splatter him?
He was a by-stander who ruined our fun for his petty idealistic reasons
No matter what he did, it was wrong to splatter him. It could be considered assault and get you sent to jail.
We splattered him to ~10 feet away. It doesn't matter.
Even though you might have not been seen, don't you think he would have felt really scared that someone assaulted him like that? How about trying to apologize the next time you see him, he could forgive you and you might become friends from this moment.