Billy knows the best kind of violence is...

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See?  I've been not endorsing violence since the early days of this comic!  And all these years later, it's still fun to see cartoon violence!  Who was Barton?  Well...
There are so many shootings, I when I first looked at this strip again after all these years, I was thinking it was David Barton who is a prominent anti-choice guy.  But I just Googled "Barton shooting 1999" and found a Wikipedia page for Mark O. Barton who shot 25 people, killing 12 of them, in a stock brokerage in Atlanta.  Seems he also killed his two children and his second wife and is a suspect in the killing of his first wife.

So, yeah, this guy was a real winner.  He killed himself four days before I originally posted this strip.  Obviously, this strip wasn't really meant to be directed at Barton, but anyone else thinking violence was a good way to deal with one's problems.