Of course you can't be arrested for murder if the murder hasn't happened yet, just as you can't be arrested for burglary if the burglary hasn't happened yet.<quoted text>
That's a trick question. If they are a criminal, sure they could have been arrested for something else before committing murder.
But the police can not arrest someone for murder simply because they suspect they will murder someone.
But you CAN be arrested for attempted murder or conspiracy to commit a murder, etc.
So criminals HAVE been prevented from killing people due to enforcement of existing laws.
Yep, that was too easy.