This is assuming that some lives are more valuable than others. Since we are all equally human (regardless of the maladies that may be present) then no group can fairly be extinguished, because we all have equal right to be alive.
Wouldn't it be nice if some humans were truly more valuable than others?
Well, no it wouldn't be nice.
But that is the only way that human extermination could be justified, regardless of the group's ability to contribute or care for themselves.