Alexander was incredibly angry, not at Cassian but at Milena for manipulating him so blatantly. She acted as if she was on his side and supported his decisions to get closer to him, probably so he wouldn't discover her moves in the shadows.
Now he looked foolish, even though he could wipe her off the face of the earth right now, he felt weak. He sighed and approached the boxes, looked at Cassian, and placed the paper in his box.
He didn't want to go along with Milena's plans, but he was forced to. If he fought her now, the family would be utterly destroyed. Maximus and Octavius entrusted him with its care, and he couldn't just destroy it.
That's why Alios stopped him earlier as well. Giving the throne to Seraphina now and starting another fight for it after the war was a better solution.
The important thing was for the family to enter the upcoming war organized and strong; otherwise, they might truly disappear.
