He said he was firing at enemy mortar positions. You know, people shooting at them

Him having such enemies is a direct and very expected consequence of him signing up to torture, kill, and otherwise subjugate the people of Iraq and Afghanistan multiple times.
The only way you can think he had any right to retaliate in general is if you think that repeatedly signing up to torture, kill, and otherwise subjugate the people of Iraq and Afghanistan multiple times is perfectly okay.