Gene thinks Finny is telling the truth about killing himself if Gene became the head of their class. What other reaction could Gene have had?

If Gene better understood his relationship with Finny, he might have realized what emotions were at play here. Certainly Finny wanted to maintain the status-quo of both his identity and his relationship with Gene. When this is altered, Finny feels like he is losing control. It isn't so much that Gene is class head; it is more about the disintegration of identity.