There are two immaterial things that Xavier considers inaluable. One is loyalty - the other is honesty. Lucas has proven time and time again he isn't loyal. Honesty was another matter - a cousin of it was truth which arrived like a kick to the gut. His face contorts into a grimace, winded by both his statement and the kick and his hands turn into claws.
No words escape him, only distraught screams of rage. The light vanishes completely as Xavier fights to break free from his grip, rip off that cruel laugh, crush his condemning throat, and smash his terrifying head into the ground. It is purely human - the truth about being changed into a monster. No animal acts out of revenge; they kill to defend themselves and to feed.
In another time and place, Xavier could recognize this, but he's too far gone now. Light isn't needed to see how contorted his face is and how tears leak from his eyes.