If you break into my home with a gun, I shoot at you and miss, and you shoot me and kill me, you would absolutely be convicted of murder or manslaughter.
The defense, in this case, relies on two claims:
1. The intruders were police officers (this doesn't, or shouldn't, give them carte blanche to shoot people)
2. The police announced themselves as police before entering, which is in dispute.
Further, the opportunity to bring manslaughter charges was never provided. The prosecutor declined to present murder or manslaughter to the Grand Jury.
The USA seems to have got itself in a very peculiar place with regards to discharge of firearms by representatives of authority. Certainly in the UK and I think on most other western countries, you don't just shoot wildly into a property because someone inside has discharged a firearm.
It just wouldn't have happened in anything like this way in most other developed countries.
Sure you would, if somebody was returning fire after having been the initial aggressor. Suppose I were to break down your door. In many states, you would be justified in defending yourself (Castle doctrine). If I return fire and kill you, I would be guilty of murder. You had a right to defend yourself, and I did not have a right to return fire.
That's the a huge point of the outrage, that whether or not the police announced themselves is in question, and witness reports vary. If the police didn't announce themselves, then there is no way for somebody in the house to determine whether it is a valid warrant being served or a violent home invasion. (Or both.)