I have read that he also impaled his own soldiers just to freak out the invaders by showing them how bad-ass he was.
Don't know whether or not that's actually true, but I can tell you that I wouldn't have wanted to mess with him.
Probably deserters. The Romans crucified them. In modern times they shoot or hang deserters. It does wonders for discipline in the ranks.
It also kept pensions low. The guy in charge of reenlistments was probably one lonely dude.