Voluntary manslaughter is considered “heat of passion” killing.

Let’s say someone accidentally hit and killed your family member in a car crash, and in your fury you killed the person who did it, that would be voluntary manslaughter.

2nd degree murder doesn’t include the element of an elevated emotional state.

I assume by charging the officer with voluntary manslaughter the commonwealth is arguing that the officer was in an emotional state that clouded his judgement when it happened.