Cleansing the most tainted and potentially most deadly of all the traitor legions makes them scum how, exactly?
Actually. They were given orders to destroy this world way before the Thousand Sons fell into the ways of Chaos
They were slaughtered because they used the psychic powers of the warp. Which the Emperor had deemed illegal. With some outside whisperings of people who had to force the hand of Mangos the Red.
So, to be fair, they came and slaughtered the entire planet (women and children also, lets not forget.) because of the word of the hypocritical Emperor. So, unfortunately, they were not this deadly traitor legion that you speak of, well when the attack happened.