It is impossible to level that judgement in society without all sorts of screwing up, and it's impossible to gauge the worth of a human being with any metric we are aware of today.
All people have use in some form, so long as they aren't actively trying to harm, and are participating in society. Even the most poor, after all, can find a home, food, shelter, in the military, where all you must do is use a gun to make a living.
The disabled must be cared for as doing so has let us have more productive, more capable, members of society. Accepting people of all forms allows diversity which causes a wealth of ideas and concepts to emerge.
And, yes, we treat those less capable as less humans. Children, unable to care for themselves, have fewer rights vs adults. The disabled as well, are forced into mental homes if they are a danger to someone, and so on and so forth.
There is nothing wrong with killing those humans who truly aren't useful, those criminals who may feel no remorse and have no ability or will to listen to the laws of society. Even though there is very valid argument to say it's impossible to say who are these criminals in the first place.