Puppies! I'm not totally sure I'm right, but I think the real, underlying problem here isn't that Killer can't join a pack, it's just that he's still in a pack and deeply confused by Duo's desire to also join this new one. I knew it was Heero on the other side as soon as Duo started knowing where everything was and I cheered. Though, does Heero know he's sabotaging Duo's integration here? Getting the criminals was nice, but the real point here was for Une to see if Duo could follow orders and work with a team, and capturing Heero's additional headcount argues that he can't. Does Heero not know that the process of getting Duo back would be "want to come with me?" "Sure!"?