Reform, and if they continue to pose a risk for society keep them in a prison/psychiatric ward until they die or are reformed.
Look at how Norway handles Breivik for example. That guy is a proper monster. But he still gets treated according to human rights, cause he is still human. We as a society should be better and more rational than the monsters that we condemn.
The only danger that creeps in here is 'who gets to decide who is useless and dangerous?' because I wear glasses and don't feel like being on the receiving end of a Khmer Rouge style microcide.
Good for you, it's not glasses this month, it's the hearing aids that indicate "useless" status and crutches for dangerous because you can use them as weapons.