One of the problems with a loosely written self defence law is that the perception of ‘threat’ can become self reinforcing.

Say you’re in a confrontation of any kind. You know that the other person can escalate to using lethal force, claim self defence, and have a decent chance of getting away with it.

That gives you the incentive to escalate first, reasoning that it’s better to be judged by twelve (or not, as the case may be) than to be carried by six.

The other person knows that you have the incentive to escalate, which which gives them the incentive to escalate first…

And it goes without saying that this escalation sequence also favours the person who initiates the confrontation (thus knows it’s going to happen), and goes into it armed and ready.