In 1.9 when Cupid told Mugm and Silva that he wanted to trade his life for Onion, Silva was filled with words of "After all of this" and you could just stare at Mugm and see him silent. Really, after everything, he was the one that suffered the most. "The Weapon", "The Aberration", he was the one that stood up on his two feet to take the risk and write Steal on that piece of paper, in an attempt to get his father back. He would have to try so hard at the events, because he was always competitive. Always a banter with Silva/Netherfall, always playing around. He was always the fighter.
When kantje says, "Why would you care about anyone other than yourself?" Mugm was in disbelief. His response of "Did you really say that?" was because he knew that it wasn't true. He just wanted to help people, help his family, help his brother. In 1.5 he knew someone had to die. Right after Mugm popped Silva's totem, Mugm could just hear the screams of Decity. And the worst part is that you could see him, looking up, wishing that this would have never happened to Decity, and then he just stares at the floor, of, "How can I protect them?", searching around frantically for an escape out of this prison. But he never wanted his other brothers to die in place of him, so he let Silva kill him, to save both Decity and Silva from their deaths. Violence was built from rage, as kantje had gotten him exiled, the place that he protected whilst fighting against Ivory. In 1.7, he was filled with rage, wanting to immediately take out kantje, but couldn't. Reason why? Boba might have been killed. He had to watch in that arena, praying that either Geo or Silva would be taken out quickly. But he knew he didn't want Silva to die. Deep down he knew that. After the fight, he tried killing kantje, but the world wouldn't let him. Everything was hopeless. He had to do his trial, doing what he excelled at. Fighting. Until facing with someone who was different; an illusion? His father. "It's an incredibly weak aspect of you, fighting everything that crosses your path, that inconveniences you." He just... wanted to protect his family, his friends, his brother that stood with him throughout everything.
--- He had to kill Gooze. Why? That wasn't them. Take him out of the suffering. already. please. "Where's silvaasaur?" Only wanting him to be safe. "I don't kill to kill kantje. I did it....for my family." Truth spoke. He finally killed kantje. Would it be over? Would they finally get the disc? They were at peace..? No. They never were, and they never have. Silva loses his memory, another person torn from Mugm's hands, his last person, his brother. He needed that disc now. He almost had it. Rin was right infront of him, kantje had lost his memory. He could use it to bring his father back. Until he couldn't. Stuck in another dimension, fighting off someone he truly cared about, his teammate. Looked just like the Silencer he had fought with. And even during that stream, when Silva was finally regaining his memories, Silva said, "I killed you. I killed you...Mugm." and Mugm was just turned around, but slowly turned towards him. Joy in his eyes, his heart, that his brother finally remembered. He got the reward he finally wanted. The disc. His father to be back. His brother finally had his memories again. They would be a family. A Callisto Family. But, of course, it would never happen. Ripped apart from his once again, his father. They had no purpose. Those two brothers. Truly a black hole. They tried so hard to get the thing that they wanted more in the entire world, their father. All of it really was for nothing. They were simply, nothing. Brought to Hell as a mean to be removed from the cruel world called "The Narrative", separated, but maybe it was for the best, maybe they could finally be free from it all. After everything. Until He arrived. The same person that ruined all of their lives. Coming back to save them both? They finally escaped though, it's all good, brought to the same place, the same place where their own father had left them. The same place where they had lost their purpose. Truly a black hole.