First degree would indicate planning and premeditation. That’s going to be very difficult to prove (indeed, virtually impossible unless said cop was explicitly looking to kill the specific guy).
Here’s the skinny on first degree murder in IL
http://statelaws.findlaw.com/illinois-law/illinois-first-degree-murder-laws.html
And 2nd degree
http://statelaws.findlaw.com/illinois-law/illinois-second-degree-murder-laws.html
Only difference is 2nd has mitigating circumstance ie sudden passion or (unreasonable) belief that the killing was justified
They have to prove intent to kill for either or “Knows that the acts create a strong probability of causing death or great bodily harm to the individual”, busting a whole lot of caps probably qualifies.
“Voluntary manslaughter” is just for the unborn
http://statelaws.findlaw.com/illinois-law/illinois-voluntary-manslaughter-laws.html
Here’s involuntary
http://statelaws.findlaw.com/illinois-law/illinois-involuntary-manslaughter-laws.html
Looks like they have a case for 2nd degree, charging 1st is a reach, I don’t know how one could say they’re weren’t mitigating circumstances.
Premeditation can be formed in the twinkling of an eye.
I believe they based this charge on the several second pause between shots.