Acting was a way out at first. A way out of not knowing what to do, a way of focusing ambitions. And the ambition wasn't for fame. The ambition was to do an interesting job.
When the habitually even-tempered suddenly fly into a passion, that explosion is apt to be more impressive than the outburst of the most violent amongst us.
In combat, life is short, nasty and brutish. The issues of national policy which brought him into war are irrelevant to the combat soldier; he is concerned with his literal life chances.