From the limited info in this post, the verdict is bogus. Yet another reason that the DP should be removed in all states. He is on death row, probably, b/c the guy was a cop. States often have increased penalties when they are killed. Of course, other factors may have come into play - race, the idiosyncracies of the members of the jury, including the power dynamic in the jury room (which I have seen firsthand), etc.
Further, if the warrant was not for his apt. then the cop was completely out of line for coming into his apt. That doesn't mean that the cop should be killed but it's just another example of problems in our justice system - just like in any other system.
If MS does have a separate offense of killing a cop, I wonder if the state even provides self-defense as a defense to the offense. Even still, whether he was using self-defense may be questionable if he didn't even wait to see if the person was really acting with an intent to inflict bodily harm on him. Further, SD only gives the right to use the force coming at you - so not deadly force if the person isn't threatening deadly force or serious bodily harm (so you can't shoot someone who is coming at you with an open hand to slap you for instance). Still, on general principles of life and fairness, it seems like you ought to be able to shoot if someone's in your house and still coming .... It doesn't make sense to me.
Removing the fact that he was a cop, this definitely doesn't seem like the type of facts that should lead to a death sentence. Adding in the cop factor, MS probably has some kind of increased penalties for that and that's probably partly why. Anyone of us could have made this same mistake.