You are not taking their soul, but their life energy.
Which you took anyway when you killed them. Kind of like the Fremen in Arrakis, they take the water from the body after it is dead, why waste it. Same with the life energy. It either dies out with the body, or it gets absorbed by the gem upon death. Why waste it.
I can't believe a moderator has such a sadistic view of things! Aren't you supposed to be a role model?
There is a book about a teenager who summoned a daedra, to asked it to make a birthday gift for his mother or something.
Long story short, the teen made a mistake of accepting a gift from the daedra which broke the summoning, and the daedra murdered the teen.
Then the daedra mused to himself that he is not letting his soul get away because he used a black soul gem, and he has another to use on his mother.
So black soul gems do trap human souls that are supposed to go free, not just the life energy.