Now I fear replies of the "But all perpetrators of Crime X can never be cured!" sort (with no supporting evidence, of course). The desire of modern society to punish everyone who doesn't live up to what is basically an arbitrary moral ideal without any attempt to help or understand the criminal is, to me, just as disturbing as the crimes they are seeking punishment for.
I find it interesting and instructive in situations like this to think about what it would take to make you or me go over the edge and just start killing people (I'm NOT justifying murder in ANY way, I'm just throwing this out there). As I said above, these people did it for some reason, despite most or all of them knowing that there was a punishment involved...what is the reason? If we uncover it, can we help them and maybe release them into society? Why is my response starting to sound like a bad paper for an undergrad philosophy course? (-:
I think that letting serial killers out of jail is a bad idea, yes. The reason why we should keep serial killers in jail, though, is not that they're serial killers - it's that, in most cases, they tend to not care and will willingly kill again.
If there is a serial killer who is provably not sociopathic and who can actually become reformed, then that person should be eventually let out of jail. That is a very difficult standard to meet.