[@mattmanganon] I mean, I'm pretty sure he wiped out the Thunder Warriors specifically to make room for the Space Marines. He wanted his reign over the galaxy to be represented by noble paragons of humanity, which the Space Marines would have been in perfect circumstances; the Thunder Warriors were psychopaths and murderers even before their surgeries to alter the traits needed for their conquest of Earth, and hence obviously not noble enough to keep around. I highly doubt he intended to ever get rid of the Astartes or the Primarchs, even if they were simply a means to an end.

Regardless, I think mutation into a full beastman is too substantial for a Primarch. Something that major won't be allowed.