An odd feeling. Not betrayal itself, that simply stung. The odd feeling was when it no longer stung. Malliston lately had found it harder and harder to feel what he knew he should. It had always been an issue. He had been told repeatedly that he was a grim and cold man at times when not being demeaned for his arrogance or pride. Now though the issue was not finding the proper way to express his feelings, it was having them in the first place.
Upon return to Stygia and Enyo, home of not only his Legion, but the center of what had been his own little section of the galaxy Malliston was briefed of the betrayal of Atlas at the far reaches of the Imperium. The idea of his brothers, at least two of them, some of the reports he was getting from his men were conflicting, becoming traitors, and another two dying possibly was enough to shock him. He couldn't believe how quickly the galaxy had seemingly changed. Over the next weeks he had come to accept this new reality, and over time the revelation or perhaps his own nature had slowly deadened his heart toward such a thing. Word of a primarch's betrayal had led to scatterings of discontent within his own domain. This again caused a sting within his heart as several of these rebellious nobles and commanders had once approached him to split from the Emperor's will and strike out on his own. They had vowed their loyalty to him and he had accepted their loyalty greedily but turned down their desire to be free of the Imperium. Malliston had believed it madness to defy his father and siblings, but now, those nobles and commanders rose up against him because he remained loyal. The thought of betraying the Imperium remained madness in his mind. Surely in the end all the traitors would meet the same gruesome demise.
As he retaliated against the suddenly rebellious forces on his own homeworld, Malliston found himself growing ever more grim and apathetic to the conflict. It was all pointless he realized. Should he fail to suppress the rebellion it wouldn't truly matter for them, his siblings would come to offer aid and the rebels would die either way. Their cause for a liberated and independent Stygia proven even further than their betrayal of him as a foolish if not completely naïve and doomed endeavor.
When some of his own legion turned against him to support the rebels, many of them natives of Stygia, some of them even his Old Stones, that had fought beside him before even encountering the Imperium, Malliston hardly registered it. As the once beautiful natural landscape of Stygia, healed after over a century of peace, largely thanks to his own efforts, turned into pockmarked and poisoned fields of corpses, he didn't even bat an eye. Everything he felt he had given his once beautiful form, his freedom, and his health for was turned against him and he could at most grunt in acknowledgment of the reports.
His loyal officers even began to doubt him after so long. The death toll on the population of Stygia and Enyo had grown to horrendous scales. HIs legion suffered rather minor losses all things considered, largely because they had begun to take the easiest route to victory by this point. Entire hab blocks gassed to eliminate a singular cell of traitors. The vast majority of Stone Warden casualties came from squads turning on one another like animals.
By this point, Malliston had lost all control of his own homeworld. The battle had spiraled further and further out of control as his normal tactics failed him. His new go-to strategy of scorched earth seeming to bring the only true victories he possessed and even then they were pyrrhic. The truth was however he could have won this battle before it got out of hand, had he only acted quickly from the onset. Instead he hadn't cared. Even now, while he was annoyed by the notion of having to ask for aid from his siblings, he found he couldn't care enough about the humiliation to stop himself.
Now Malliston, and his sons, in their scarred and crackling armor covered in chemical burns, had to reach out to others unless he share a punishment like his brother Korteaz. Malliston of course could only see one true strategy to destroy his traitorous sons and subjects and return to support his fellow primarchs. He no longer cared at all for the people there. The only things topping him being the destruction of himself and his Legacy for the act of cleansing his planet.
So the message was sent, it's security rating restraining it's access to only his fellow primarchs or the most high ranking of Imperial officers. Another primarch could make the call for the fate of his home. Burn it, save it, as long as the final decision, and possible retaliation from the Emperor fell somewhere besides himself and his sons, it didn't matter.