I would have to say that you cant use today's moral and ethic standard and compare them to
40k, there standard of whats right and whats wrong could be complete different then today's standards and of course as was stated earlier what one faction deems justice and right another views as the most vile evil. And also as stated earlier are view of morals is human and there fore how could you compare them to any other faction except the
IoM.
Orks, kill and wage war off instinct nothing more then an animal does they dont have human morals that killing is wrong perhaps not killing to them is wrong.
Nids, are controlled by a single powerful mind that possible knows what it is doing is mass murder and genocide but again perhaps this is right for the race and not killing is wrong
Necrons, where once humans
IIRC, thus what they do could be sayed to be evil and wrong by human standards unless im wrong that they where not human
Chaos, knows that killing, murder, pain suffering etc. is what fuels them thus its right from there perspective, wrong from humans and evil
And the list goes on
id just be repeating myself, the
IoM by today's standards is evil and wrong but there awesome

.
Also about killing depending on who you are its wrong or justifiable or maybe necessary evil it all come down to the person.
And I think a good faction could survive for a bit it mite even get the story line advancing for a change