Personally, if I had purchased that gun I would not accept the reward / offer.
1st reason: I would not ever be happy knowing that I had purchased a murder weapon. One used in self defense, not a problem, but used to actually murder someone? No, never.
2nd reason: It is the obligation of every citizen to assist law enforcement in whatever manner is best suited to the individual, to put dangerous felons in jail, when requested. The LEO's in this case are making such a request. I'd be glad to help them, and no amount of money would be required, only the personal knowledge that I have assisted them in putting the worst of our society away.
Ron