If mentally handicapped people are "useless to society" then so are those with physical handicaps including obesity, missing limbs, the deaf and blind, people suffering from terminal illness, the elderly, children between the ages of 1 and 5, George Bush (both of them), jews, gypsies, the poor...hmm this is starting to sound fimiliar.

Anyways you say its the tax payers who end up paying for these people and they should be killed? It does cost money for lethal injection you know. Though I suppose you think we should just throw them all into camps for now until theres room in the gas chamber for them huh?

OR We could stop wasting hundreds of billions of dollars on these wars on drugs and terrorism and put that money instead into medical care and research and education. This way we wouldn't have to kill anybody.
By the way many mentally challenged people are fully functional members of society and likely contribute just as much if not more than me and you. If you think about it most people never really "contribute" anything do they?