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,"
Including made-up, forged and totally illusionary evidence. Just because they are "available". And if they aren't, Police will make it so.
So basically you want a witch hunt where "murderer" is guilty by default and you just make the evidence to prove it if it doesn't exist. Piece of cake for Police and a common practise in every country where illegally "collected" i.e. made up evidence is allowed in Court.
Total dictatorship, basically, and you call that "sensible"?
Yes, it makes sense if your only goal is to put people in jail/execute as much you can and being actually guilty is totally irrelevant.
Because there isn't and never can be any limit how much illegal evidence you can make if you want. Some picture manipulation? Piece of cake. Forged fingerprints? Who cares? Traces of DNA in wrong places? Totally trivial.
Forging evidence is so trivial that literally anyone can do that.
The second it's illegal but still causes convictions in court, it will be done in large scale: Prosecutors, best friend of the Police, want convictions and actual guiltyness is totally irrelevant to them.