For Celestia's sake, you people! Revenge in blood?! We can't let our Ponymons be turned into killers!
Maybe let the Manticore rough him up a bit, a little catharsis to ease his pain after the loss of his parent, but don't let the poor creature kill. That's a road no pony, or manticore, or any creature should be allowed to go down.

The mob is clearly already decided, but if I may say one thing: don't let the Manticore go all the way. Pull him back before he does any real damage. If nothing else, we can use this as an interrogation technique to get some information out of that goon.