The issue is with the initial search. They can only search after they have the warrant, and they allegedly made the arrest knowing they would find a gun, and then got the warrant, and then “found” the gun again.

Even if they claim that as long as they did find the gun, it means they were right - the problem is that, had they not known about the gun, they wouldn’t have had grounds to arrest him, and therefore they wouldn’t have a conviction. That sounds fine as long as you do find a murderer, but not for everyone else they search like that with no warrant.

Even assuming the gun was real and not planted, the legal issue is that they can’t be allowed to fish for anyone they want, even if they happen to land on the right guy - after countless other innocents. Forcing them to let an actual murderer free would, presumably, push them to think twice before violating random people’s rights.

And after that, they have to prove somehow that the gun wasn’t planted while they had the bag and the owner couldn’t check on them.