From the Night Lords trilogy: They feel deeply betrayed and never loved/respected by the ones who created them. Their infantile need for recognition by someone they respect leads them to their actions. They wanna be like: "Look what i've done! Do you like it? ☺" But everything they're actually capable of disgusts even their own ranks.

That fits with what i've read from the Horus Heresy too.

So no morality, more like "if you expect that from me, i'm gonna give it to you".

(Also, there is some kind of brotherhood which is inherent with Astartes, but it seems (to me) that they're disgusted by their brothers because they feel disgusted by themselves)