There's truth to that, of course. There is a chance he can reoffend. I think the fact of there being a "11% recidivism rate in 35 years" among mentally ill is a pretty good bar, though. Compared to studies that show 68% reoffending rate within just 3 years for mentally sane criminals.
You can't just throw all mental illness related offenses in a bucket together and compare recidivism rates. This wasn't just some run of the mill offense that might be related to mental illness. Some mental illness can be very effectively treated, it depends on a lot of factors. A more apt comparison would be to look at the recidivism rate among those convicted of child molestation or rape. There is really no "cure" for whatever drives people to molest children, and recidivism rate is around 45% within the first 5 years, probably much higher in the long run.
Now add in the fact that there's no way to ensure that this guy continues to take medications or seeks additional help if things deteriorate, and what you have is essentially a ticking time bomb loose on the streets. It may never go off, but I wouldn't count on it.
Those opposed to his release keep harping this. It's been mentioned at least a half dozen times in this topic. Let me perhaps be the first to say, I would not feel uncomfortable.
Then's fine for you, but I suspect you're part of a small minority. Remember, this is a person that has shown that he can snap with no provocation whatsoever and go completely nuts and stab / behead you. There is no way to make sure he's taking any meds (and no way to know if the meds will even work effectively for him or not), and there might not be any indications that things are going wrong. Even if there are and you notice strange or alarming behavior, there's nothing you can do about it, you are stuck living next to a potential time bomb.
This isn't just a case of stigma associated with mental illness coupled with an irrational fear. This is very rational legitimate concern and fear, and I for certain would not want to have that kind of danger near me if I could avoid it. Would you leave your child alone with a known child sex offender after they've been released?