You're absolutely right. Completely different scenarios. The black man killing a cop is doing it as a reactionary revenge killing against (what is likely to be) an unintentional death. The cop is routinely forced into heated sometimes violent scenarios as a result of his profession and is expected to be a 100% zero-error machine when making decisions, put his life on the line for what barely qualifies as a salary, and live under constant scrutiny. When the officer makes an error in judgement he's got watchdog groups fanning flames and calling for his death. Someone revenge killing a cop isn't exactly likely to undergo the same level of scrutiny.
Here's what's the same. They'll both be prosecuted according to the law (if they can be found, in certain scenarios).
Now ask this. Since the officer(s) were fired, an investigation is underway, and even if found guilty of the highest possible offense, one side of the line still won't be happy: why is there rioting right now? You know, since the legal wheel is rolling, rational individuals would be waiting to hear the results of the trial, right? Why is there rioting? You literally can't (shouldn't, anyways) arrest someone until you're certain you have enough evidence to make a case lest you risk them walking. So why is there rioting, in your mind, right now? Why would someone kill a cop right now?
Have fun fanning your flames. Hope you don't end up in some friendly fire. Not likely since you're probably some upper class white dude far away from the chaos you're creating.