As an Anorak, I have to say it's your kill.
If your figs' deaths could be attributed either to yourself or to nobody, then you could have your figs all commit suicide and deprive your opponent of any remaining kills. That would be utterly underhanded, and worthy of some serious Hammer Of Discipline action, but still, it's a possibility.
Hence, the solution is that if one of your own guys dies, the credit goes to your opponent. If you're playing against 2 other players, then you've got a dilemma on your hands (assuming you're truly responsible for your own fig's death). In that case, I'd say you lose one of your own kills, rather than giving a bonus kill to one or both of your opponents.