He should have paid more attention to the signs. It had been obvious that Lysandre was losing his sanity. Sycamore had blinded himself to the fact because of his emotions. Lysandre had always been a man with great passion to help the world. It had somehow gotten to the point where Lysandre had talked about cleansing the world of filth. He might have been able to save Lysandre from himself, but Sycamore had been much too late. Lysandre was dead, and Sycamore shared some of the blame for it. He was kept awake at night with questions of what he could have done differently.
Sycamore learned that Lysandre had died from Sina by Holo Caster. He had felt numb, barely managing to thank Sina for telling him. The silence after Sina disconnected was cloying. In a rage, he had thrown the device to the wall, but it had only bounced off without leaving much of a dent. He remembered that Lysandre had been the one to develop the device, so went and picked it off the ground. He was overcome with emotion by holding a reminder of Lysandre in his hands. He had cried then, alone in his apartment.
There were still things to do, so Sycamore buried his feeling of sadness away. He challenged Serena to a battle, thanked her for what she did to prevent Lysandre from going through with his terrible plan. He had told her how he placed some of the blame for what happened on himself. Serena tried to tell him it wasn't his fault, and Sycamore hadn't the heart to disagree with her. He organized a parade to celebrate Serena and her friends as heroes. It helped him discover that he still had a life to live.
Losing Lysandre had felt like losing a piece of his soul. Sycamore had loved him, still loved him even now. Lysandre might have been planing the unthinkable, but it had come from a place of desperation. It broke his heart to think Lysandre wasn't able to see any other way to keep the world from ruin. He would carry on, live to help the world not become what Lysandre was afraid of. It was a way to honor Lysandre's memory, not let his death mean nothing. It was a daunting task, but Sycamore loved him too much to do anything less.