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.
Self defense was actually supposed to be considered when we're at the point of being oppressed or being at the point of taking advantage about. Probably retaliating based on the circumstances of the situations. Just as you said, it doesn't feel good to use a heavy tools such as firearms on someone with unarmed but something we must consider is that assuming the attacker is physically fit to stand unarmed fighting with a less power victim who's armed, should he/she not used the weapon for a self defense? Let's also understand that even the unarmed has the potential capacities to take lives off with their bare hands so let's not have that insightments that only armed people are liable to destruction.
We can only consider the capacity of the nature of tools we uses to defend ourselves depending on our personal believes that we can handle the tussling situations such as de defending ourselves with a minor tool for arms or unarmed else we might end up being defenseless and helpless.
Some persons in the situations of being victims are actually heartless and abuses the law of self defense by abusing the right of being defense full.