Some people can, and some people can't, shake off regret. Even over something that is stupid to regret. It's something I'd wish on no-one, to never be able to shake off demons. Anyway, moving on... I hope you never have to find out how you'd react to killing someone.
As usual Wikipedia is a good first place to go on this issue:
http://en.wikipedia.org/wiki/Self-defence_in_English_law
The following paragraph is particularly revealing:
[i]"The defence of self-defence is one which can be and will be readily understood by any jury. It is a straightforward conception. It involves no abstruse legal thought. ...Only common sense is needed for its understanding. It is both good law and good sense that a man who is attacked may defend himself. It is both good law and good sense that he may do, but may only do, what is reasonably necessary. But everything will depend upon the particular facts and circumstances. ...It may in some cases be only sensible and clearly possible to take some simple avoiding action. Some attacks may be serious and dangerous. Others may not be. If there is some relatively minor attack it would not be common sense to permit some action of retaliation which was wholly out of proportion to the necessities of the situation. If an attack is serious so that it puts someone in immediate peril then immediate defensive action may be necessary. If the moment is one of crisis for someone in imminent danger he may have [to] avert the danger by some instant reaction. If the attack is all over and no sort of peril remains then the employment of force may be by way of revenge or punishment or by way of paying off an old score or may be pure aggression. There may no longer be any link with a necessity of defence... If a jury thought that in a moment of unexpected anguish a person attacked had only done what he honestly and instinctively thought was necessary that would be most potent evidence that only reasonable defensive action had been taken."[i]