This is a complicated answer. As you say, there are two types of punishment, the harsh definitive kind- death penalty or roting in jail; or the rehabilitating kind. I've already said I don't think the former accomplishes anything and the latter requires that the rehabilitated person be sincere and repentant in what he did. He already offers Hawke his life as penance so he does mourn the death of those he sacrificed but I doubt he regrets it as he sees it the only way. Then there is the matter that it's up to you or Hawke to enforce the rehabilitation in which it impossible as it's up to Anders to rehabilitate himself. It is beyond regular means unless one is a god or mind wipe Anders to be a drone to your will to repent. So yes, I am arguing that the only recourse is to let Anders walk scot-free because judging him is only delusional self-righteousness and it's beyond your means to actually judge him unless you kill him. And as Varric tells Cole in his personal quest, killing won't make the pain go away. As I see it, if you kill him then it's done. If you let him live, you believe he won't be a monster and will live the rest of his life "good". It may not be a life of pacifisim, but of a way something anders and justice can die with out regrets and either you come to terms with that and agree with it or you've deluded yourself and let a monster roam free. Death will come eventually, whether it be by living as a fugitive or as a grey warden. But if you choose rehabilitation, he will be "free".
Clearly, we are going to have to agree to disagree because Anders certainly cannot walk scot-free. I don't agree with your analysis or your dichotomy of one's mindset if one didn't kill Anders then ("either believe Anders isn't a monster or you let a monster roam free"). I also do not believe judging Anders is an act of delusional self-righteousness nor that it is beyond our means to judge him without killing him then and there but I don't want to get into it now, "it" being the appropriate sentence for an out-of-control spirit-man terrorist-advocate, if the death penalty is just or not in some cases, etc. Just because we might not be able to give him the most appropriate sentence doesn't mean he can go unpunished. I absolutely cannot wrap my mind around that and since that's your stance, I don't want to discuss it any further.
I chose to let him live. And I chose to not let him walk scot-free. Like you said, since Anders is not longer capable of fully controlling himself, if he goes free, truly so, he will pose as much danger to others as he would to himself (probably, or maybe he'd eventually kill himself in an act of delusional self-righteousness). That is one of the reasons I am against him being free. The second reason is to try to rehabilitate him. We're talking about fiction here. The Rite of Tranquility was said, if I remember correctly, to be irreversible. Then look what happened? And the third, is of course for him to answer his crimes, one way or another, in the vein of the Recruiting Inquisitor, or making Loghain do the Dark Ritual and/or the Ultimate Sacrifice - something productive. One doesn't need to regret what one did to recognize it was a horrible deed that needed to be answered for. For example, over the centuries, what the Grey Wardens did or could have done to end a Blight.
Anyway, that's all I have to say.