Explain what Henry is going through when he begins firing and becomes " not man but a member " Why doesn't he run right away?

When Henry begins to fire, he finally becomes a part of the troop. He is no longer concerned with himself as an individual, but rather takes his place alongside his brothers in arms. From the text, we can infer he is feeling confident.... selfless.

He suddenly lost concern for himself, and forgot to look at a menacing fate. He became not a man but a member. He felt that something of which he was a part--a regiment, an army, a cause, or a country--was in crisis. He was welded into a common personality which was dominated by a single desire. For some moments he could not flee no more than a little finger can commit a revolution from a hand.