What is the rationale of those who want to make self defence illegal?

Considering this on self defence and the way the court look at it. To this I think some people will consider the measure of defense against the strength of attack. Like if it is assault done on someone with mere plastic chair and the defender goes on to use lethal or gun to retaliate, this is not to be considered as self defense. Moreover if the attacker dies from the lethal weapon of the defender, it becomes a crime. I think this is the rationale behind the illegality of self defense. It should be measure for measure and not taking the opportunity to cause bodily harm while the attacker's effect wouldn't inflict such injury.