Bill gets shot in the head because the cop doesn't like him.
The end.
You greatly overestimate how complicated corruption has to be.
Bad cops shoot people for no reason all the time, relatively speaking, so this story has plenty of precedence, unlike the above story, which is pure conjecture.
Someone killing someone they don't like, purely because they do not like them, and then getting away with it because they hold a position of influence or power of course happens, and is nothing to do really with whether or not anyone involved is a cop. That situation has been playing out a lot longer than there have been cops in existence and also has very little to do with the current discussion.
edit -
also, in the simple version of your story, where 'a cop shoots someone they don't like, the end', the story is actually too simple to be about corruption.
You need the rest of the story to know if that particular cop is corrupt. As it stands it is just plain murder.
Now if the cop doesn't then hand themselves in and starts planning on how to get away with the killing, then you are into corruption, but also the story gets more convoluted.
Both stories are about things that occur in the real world, however one is about the subject of privacy and surveillance and one isn't, so given the thread I think the one on topic is possibly more constructive.
Yet there are no examples of the story being told in the above comments. It's literally fiction, and it hurts the conversation.
I assume that cops while having great leeway in how they frame their story, still have to explain such incidents and have a risk to their career if they don't do a good job of it.
So at the very least the cop has to hate Bill enough to go through the pain of explaining away Bill's death and dealing with any administrative consequences.
Forget the state; possession of marijuana is a federal crime rendering the possessor eligible for jail time in all 50 states, including the states where it is regarded by the general public as "legal".
This strikes me as an incredibly dangerous development that most seem to ignore. Selective enforcement of laws broken by everyone, all the time, is just as terrifying as the loss of privacy.