that's interesting, when law enforcement investigate a crime don't they look at motive?

What if law enforcement doing the actual investigation is the one who benefits from it/holds the motive?
I mean, imagine a small town sheriff's wife being murdered... and the sheriff gets to investigate and declare it a murder/suicide, and then he gets to enjoy the $1mil life insurance money with his long-time girlfriend.
oh, and his brother, the editor of the local newspaper, writes articles in the paper declaring how brave the sheriff is for "overcoming tragedy with such bravery!"
Then, the sheriff's other brother, the mayor, writes a lot of new laws giving the sheriff more power to crack down on local criminals.
Then, the sheriff's cousin, the town accountant, delivers a 50% increase on the sheriff's budget so he can get new squad cars, pay himself more overtime, etc.
Etc. Etc. Etc.