Did they yell "Police!"?
Because if they did that, and then you see police busting in the door, and you still shoot at them...you get what you deserve.
If they didn't yell, I can see shooting at an intruder. But still, most people would have a moment of hesitation (and not shoot) because it looks like police.
Either way, the cops are likely covered under the law because of the Warrant.
But like I said before, it's hard to say without more information.