Re: This seems similar to the global snooping (encryption) issue.
"In any sensible country they'd first prosecute the murderer, using *all* the available evidence, however obtained, then prosecute the incompetent/corrupt policemen."
The trouble is that even in a "sensible country", the second of those two things almost never happens.
You'll occasionally get a high-profile prosecution of some especially blatant abuse of power, and all that tells the authorities is that when they abuse their power, they shouldn't do it so blatantly.
There's a big gap between impropriety that should cast reasonable doubt in the trial of the alleged murderer, and proof of deliberate corruption that could lead to prosecution of an investigator. Without a fruit-of-the-poisoned tree principle, the police are free to operate in that gap.