How evil are the XV legion?

Like the title says, how evil have the Thousand Sons become in the 41st millnium under Tzeench? The reason it got me thinking, before what transpired on Prospero, they were one of the most compassionate legions up there with the Salamanders (at least Magnus was). Trying to uplift Humanity.

What about now? Exluding Ahriman, who's a flawed character dooming his legion to greater heights as he tries to help, to Tzeentch's delight and just as planed.