I was probably thinking of some no-knock raid or other, that seems to be the most common way people are acquitted of police killings as self-defense.

Here's a good example (wiki) of a self-defense argument succeeding for a police killing, but the pigs totally killed the guy a decade later as revenge:

In 1985 Penn's vehicle was pulled over in the Encanto area during a traffic stop by San Diego Police officers Thomas Riggs and Donovan Jacobs. Civilian police ride along Sara Pina-Ruiz was in the squad car when a physical altercation ensued resulting in Penn shooting and killing Riggs with his own service revolver, shooting and injuring Pina-Ruiz. Penn also shot and wounded Jacobs and ran him over with the police squad car while fleeing the scene.

Penn argued that he feared for his life and acted in self-defense after being attacked and beaten. Penn was acquitted in 1986 on the more serious murder charge for killing Riggs. In a second trial in 1987 Penn was acquitted of all lesser charges.

Lots of extenuating circumstances with this one (wiki) don't think anyone died, but damn:

Larry Davis (May 28, 1966 – February 20, 2008), later known as Adam Abdul-Hakeem, was an African–American man from New York City who gained notoriety in November 1986 for his shootout in the South Bronx with officers of the New York City Police Department, in which six officers were shot. Davis, asserting self-defense, was acquitted of all charges aside from illegal gun possession.