Why are we so eager to work with Malcador after he psychically attacked Angron for raising his voice at the council?
A few reasons. I'll open with the serious ones.
1. Angron is not really a man to hold a grudge over something like that. Given that he once threatened Fulgrim into compliance by saying he'd kill a thousand of his sons and defile their corpses, it'd be a bit rich if he did. That, at least, is a trait he shares with Malcador.
2. The political benefits serve Angron's aims well. He advocated for rule of law at Nikaea, for the rights of the common psyker and the rights and institutional legitimacy of the Imperium institutions. That Horus can shield the Night Lords from consequences for their crimes in the name of his own position will not sit well with him, and Corax never
did get the opportunity to avenge his sons for Horus's mishandling of them. Perhaps with this grief given to the Warmaster, Corax might see them as somewhat more even.
Then again, maybe not.
3. It would
work. More than any of the other options mentioned, Malcador is
effective. He'll have had contingencies for the Night Lords for centuries and his own personal psychic might is second only to the Emperor and perhaps Magnus — and his knowledge far surpasses the latter. If Malcador
buys in — and I've laid out earlier why I think he would — Curze and the Night Lords are dead, you just haven't deigned to inform them yet.
4. This one is a bit of OoC reasoning: Malcador, for the first time in millennia... is not the Emperor's unquestioned right hand and most trusted. That's an
opportunity. As I mentioned earlier, perhaps Mortarion is not the only person who looks at Angron and sees the Emperor. At the very least, the old monster will be greatly amused by the gall.
5. This one is purely OoC — Malcador is perhaps the most interesting non-Primarch character in all of 30k (and more interesting than most of them, tbqh), and we get vanishingly little on him in canon beyond an audiobook that is, to be frank, not very good.
The fun reasons:
1. Dad said no, time to ask Other Dad.
2. Oldest Man Genocide's magical kung fu has gotta be super sick in a fight scene.
3. Malcador spending a solid two minutes doing the evil old man cackle after Angron asks him will be great.