I think there is some evidence that shows that justifiable homocides go up, but even that is debatable.
I think people have to be carefull about getting hung up on whether or not "SYG" laws do or do not deter crime - because SYG should not be thrown into the realm of either beneficial or detrimental to society as a matter of public policy.
It is a basic human right to protect oneself. If, in some people's judgement, that happens to be detrimental to society as a whole - well that's too bad.
Some view of what is ideal for an overall society should not and according to our constitution cannot trump individual rights.