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I've always pictured them from the perspective of what it would have taken for them to be offed by their brothers.

The first one would be unsuitable to lead a large army, for tragic reasons. He received his troops and became a great warrior. The exposure to conquest caused great empathetic traits to emerge, making him into some sort of galactic civil rights leader. Planets brought into compliance often turned self-sufficient and resisted the Imperial yoke within a few years, and the Imperium could not sway the convictions of those who heard him speak. When his brothers went to him about it, he was steadfast about humanity's right to self-determination, and how the great crusade must hold our greatest ideals as goals. He published books on the matter, which were widely distributed across the galaxy.

He was then executed, not by Russ, but by some symbol of authority like the Arbities. All of his writings were censored. He said something memorable when he passed, and the Imperium has spent 10,000 years trying to erase people's memories of it.

The second one would be the original Primarch to fall to Chaos. In his case, there's some kind of primordial relationship with Chaos, not connected to worship of a God. His legion became warp infused, with units picking up special traits that assist them in battle. Compliance campaigns become effortless, with the legion attacking worlds and receiving no losses. Battle computers on the ships cannot adequately model what is happening on the field, and it sets off alarms.

The Emperor recognizes the true significance of this and the threat it posed. He orders Russ to take him out. By the time they face off, the Primarch has become some kind of walking blasphemy, living in in the reality and the immaterium. Russ manages to destroy his body and the rest of his legion, save one small squad that escapes in a large ship that becomes a space hulk. It contains records of the legion, along with the (long deceased) bodies of the survivors. Any attempt to molest them causes them to instantly regenerate at full troop strength, making them almost unkillable. Any kills they make cause other members of their legion to be resurrected, without knowledge of all that has past but with a need to destroy in order to stay in the material world.

It would have to be going over to the Xenos side, wouldn't it?

Somehow abandoning Humanity and actively working against it, and the Emperor?