*UNEDITED*
From the very moment they met, Akito already saw her fragile side. He knew her weakness and fears better than anyone. He found out accidentally, but it doesn't matter how it started. What matters is that he can be a pillar of strength for her now.
"I can't be with you. Everyone's going to get hurt. Chauncey isn't going to be the only one. ..that's why....."
Akito shook his head. From the start none of those things mattered to him. He already decided it then.
Back then he decided he would stand by her side no matter what happens. No matter how many times Setsura pushes him away, he would be there for her. Besides he is very concerned about this matter. He saw from that moving picture box what happened - that explosion. The affect it has on people's bodies, it looks similar to something that happened in the town. Back in his era, people's bodies turned purple after that explosion. 'Only some ended up affected, some came out with normal injuries.'