IMO if a person is diagnosed by professional and trustworthy doctors to be sane following treatment then how can we disagree. After all they know better than us in this matter. They are qualified and we are not. it was they who judged him to be insane in the first place. And it's not impossible for a person to get well following mental diseases.
However I believe it should be then upon his doctors to take the blame should any tragedy,God forbid,happen again. It is after all their duty to ensure no sick person capable of such mayhem is set fourth into society unless absolutely sure he can now handle it.