What concerns me is burden of proof. In order for the state to prosecute someone, even if they've been caught in the act, there is a level of evidence that is required in order to successfully do so. What worries me is that you can end someones life on a suspicion, The whole burden of proof thing is gone.
Whilst self defence is one thing, shooting someone for throwing popcorn in your face is not self-defence, and claiming you believed you were in actual danger is highlighting the error in your own judgement, especially if you invoke an entirely dis-proportional response to it.
The whole reason that Americans have the right to Trial by Peer, a right which is even more important than bearing arms in my opinion, is to prevent this kind of Judge-Dredd mentality.