I think they are shown as being weak not because they have killed, but because they killed, WHEN THEY DIDN'T HAVE TO, and because they assumed their judgement was superior to the justice system they are nominally charged with upholding. Once you've gone that way it's ahrd to actually BE a hero with the realisation that there IS a societal framework they're operating in.
"I hate having to have someone killed. It's like announcing to the world that you can't handle a problem... creatively." - Lex Luthor - Lois and Clark