Putting my own personal opinion aside for the time being, our law exists to try and give people enough room to apply self-defence to keep themselves safe. Any more than that is a person taking revenge, applying punishment or just plain assaulting an attacker - in a sense, two wrongs do not make a right. Our law discourages burglary because we are allowed to do what is necessary to ensure ourselves and our possessions remain safe - simply put though, revenge and punishment are a job for the police, not for us to apply 'vigilante style'.
If a person breaks into my house and I beat him down to the ground and I have two clear options before me, one being to incapacitate and neutralise the threat and the other to just outright kill him, why kill him when incapacitating does the same job? It makes murder not justifiable in this instance - why kill the intruder when we can keep ourself equally as safe restraining/incapacitating him?
Remember why the law exists in the first place. It is there to ensure people are safe, and as such people are allowed to do what is reasonable to maintain there safety, UPTO AND INCLUDING killing the intruder if that is what is seen as reasonable at the time. If killing the intruder isn't necessary and is above what is seen as rational and a reasonable amount of force at the time, then to kill is to subject the intruder to your own malice.
Morally, you're exactly right. In my opinion, law aside, if a person breaks into my house they forfeit the right to protection. They are endangering me and I would feel no symapthy at going completely overkill and outright killing them.
Legally, this is a different case. Laws exist for the protection of ALL citizens, and just because they are breaking one law doesn't mean you can break another law when it is NOT NECESSARY to do so.
Our self-defence laws are reasonable in the sense that any victim of an intrusion or assault can do whatever they see reasonable in doing to protect themselves and fend off the assault - which in reality, is what matters - taking care of yourself. Going overboard out of anger, revenge, or for any other reason above simply what you see reasonable is then doing what is NOT needed - if your intruder is already unconscious before you, you're safe for the time being - stomping on his head multiple times is not required and as such an act of your own malice. See where I'm coming from?
I'm a hypocrite in that I believe the self-defence law here is perfect in a legal, moral sense - allowing us to treat any intruder with deadly force without consequence is a little more anarchic than I would like, and the chance to abuse that is definitely there - but (and here comes the hypocritical part) I am very morally unbound in myself and I feel I don't care what happens to an intruder whether I go overkill or not, I'm just scared of the legal consequences, not the moral ones.