a banshee, charger or urbanmech. Better yet install a double seat in Hep's urbanmech and have them ride together.
If you want him to literally die in a fire, i recommend an older Centurion or Sentinel. They are notorious for cooking off ammo.
Fuck it, give him something big with jump jets. Take full advantage of his talents while breaking his spirit by making him kill all his friends.
I think he did for his first command, but not after. And I did find out about the Bondsman issue, so I simply set his salary at 0 and changed his rank to Bondsman as a custom entry in the rank tree.
Now that I've gotten you to put an enemy in one of your assault mechs, is there any way in MM for him to betray you?