What causes the sniper to risk being shot by a machine gun near the end of "The Sniper"?
I think the general character trait that causes the sniper to investigate the Free Stater sniper is curiosity.
He wondered did he know him. Perhaps he had been in his own company before the split in the army. He decided to risk going over to have a look at him.
The Republican sniper is genuinely curious about who the other sniper is. Because the war that is happening is a civil war, there is a real chance the sniper might recognize his opponent as a former comrade.
I think tied to the sniper's curiosity is the fact that the sniper respects his opponent.
When the sniper reached the laneway on the street level, he felt a sudden curiosity as to the identity of the enemy sniper whom he had killed. He decided that he was a good shot, whoever he was.
The sniper kills three people in this story. He kills the old woman, the man in the armored vehicle, and the enemy sniper, but he only wants to investigate the sniper. The Republican sniper recognizes that his opponent was a skilled shot. I think going over to the body is his way of paying respect to another skilled warrior.