Just to look at this from one step further back, let's accept the premise. Let's imagine that there is a type of crime that is 1) horrible evil irredeemable universally-agreed-upon bad, and 2) could somehow be prosecuted with 100% accuracy and 0% bias. Yes, even then, those criminals deserve a right to vote.
Do you they're going to like vote themselves out of jail? Vote to make murder legal? What exactly are you afraid of.
Realistically they'll just... vote just like anyone else. They'll help elect city councilmembers they think will better their hometowns, and presidents they think will best serve the country. They might even vote in their own interests! To reform prisons, fund rehabilitation programs, and outlaw predatory practices by telecoms. Are you saying you don't want any of those things?
And even if there were one of those super-duper-unambiguously-evil totally misanthropic death-row convicts, who's scheduled to be execute the very next day and just wants to sow chaos and watch the outside world burn however they can... what's the worst they could do, vote republican?
Taking people's rights away isn't bad because it might happen to someone you like, it's because taking people's rights away is bad.