"Had he and I but met (1)
This desire for an alternative meeting between the men is our first clue that the speaker is attempting to take control of the events in his mind in order to change the outcome. Maybe he feels so guilty, he's literally wishing for this alternate reality to come true.
I shot at him and he at me (7)
The matter of fact language masks the speaker's guilt just as the understatement of the entire poem masks the traumatic event. He's basically saying, Yep, I shot the guy. What of it? while thinking, Good grief, I shot a guy.
"I shot him dead because— (9)
Hardy uses punctuation to imitate the speaker's confusion. The speaker literally stops himself midsentence because he realizes he has absolutely no clue how to finish the sentence in the first place. And if you've got no good reason to kill a man, well, then that means you're guilty. Harsh, but true.