They probably have a different definition of what is "necessary" than you. If waiting for further crimes to be committed makes the case more of a sure thing in their eyes) then it's necessary. Or if it gives them more serious crimes to prosecute or a higher up person to prosecute. I believe this is common in organized crime investigations.

> Law enforcement shouldn't tolerate crimes being committed on its watch any longer than necessary.

That is simply not how it works and for good reason. If it were, imagine the consequences. Most charges would never lead to a trial or a conviction because law enforcement would have acted on the very first sign of criminal action instead of investigating further or working their way up the chain. We'd only end up arresting low level foot soldiers and the higher ups would get away every single time.

The wheels of justice move slowly, and for good reason.

I'm confused what your point is? Is it when cops/prosecutors build cases that take time they are tacitly saying the crime is victimless? Do you think law enforcement should only target street level dealers/car thieves because to spend time building cases against drug distributors/chop shops renders the crime victimless?

OK but what if that serial murderer were running for president? It would be wrong to deny the people their chance to be governed by a gleeful psychopath.

As HL Mencken once said 'Democracy is the theory that the common people know what they want, and deserve to get it good and hard.'