Arsenic and Old Lace

I had seen the play before I saw the movie and I enjoyed both, but I think what has interested me most in both versions is how the characters (specifically the aunts, Jonathan) don't think what they're doing is wrong, but they think they're doing these people a favor by killing them. Teddy's character can't really be blamed for what he did, and although the aunts ultimately end up in an insane asylum, I don't believe that they didn't know what they were doing (or allowing Jonathan to do). And if Abby and Martha are older single women, where is their justification that older bachelors should have their life ended, but they should not? How do they know the men aren't also happy being single?