We're talking about viewpoints, no? Dylan Roof is not going to kill anyone else outside of prison (and probably not in prison). So if you want to take away his right to vote, it's because you disagree with what that vote might express, isn't it?

Otherwise, why cite him or other people as reasons not to let prisoners vote?

If that's the case, I refer you back to the OP, where I addressed this when I wrote about the way this rule can be used to prevent people from challenging unjust or inhumane laws.

I'm much more worried about the tens of thousands of people who aren't serial killers and are behind bars because they made a bad choice under hard circumstances, than I am about a few sensational offenders who garner all the headlines and attention.

And if that wasn't what you had in mind, then I would also refer you back to the OP, to the sections where I talk about why disenfranchisement is disproportionate and ineffective either as a deterrent or as a means of rehabilitation.