In the end, it is Dunn's admitted actions after the shooting that convict him for me. They are clear actions of a person who believes that they have done something very very wrong.
Cops and soldiers who shoot people who were actively trying to kill them can still be traumatized by it and feel awful, it can scar them for life and shut them down emotionally. It doesn't mean they weren't justified in what they did, it means that killing people is traumatic, and nearly being killed yourself is traumatic.
And they are people who not only went into the line of work they were in understanding that they may very well have to do that, but they also had (typically) the complete support of their government, their employer, and the public for doing what they did.
Yet they are still traumatized more often than not.
Now, Dunn on the other hand had no expectation he'd be in such a situation. It came completely out of left field. He thinks he's going back to the hotel with his fiancee to have some wine, and that he's made a normal, polite request. A common courtesy. Now he's having his life threatened and it gets to the point where he believes he's about to die, he then has a gallon of adrenaline dumped in his bloodstream as he reacts and saves himself.
You don't think he could be racked with guilt and self-doubt even if everything he said is true and he was 100% justified? If you think that, your understanding of human beings is sorely lacking.
Innocent people call 911 EVERY time.
Got any evidence for that? Some sort of study?
Ever hear about how many rapes go unreported? The person who got raped is innocent, and yet they don't always call 911.
Same with abused spouses not calling 911. Whether they hit back or not.
There are also a shit ton of assaults by strangers on the street, like knockout attacks etc, where the person doesn't bother to report it because they're in a big city, they know the cops will never find the guy, and it's just a hassle.
The only plausible reason for Dunn not dialing 911 is a conviction on his part that he had done something dreadfully wrong.
Is it now?
See, most times something like a self-defense situation happens the person does call 911, that's true. But most times it's sort of a moot point. They're still stationary and the cops are coming and people around know them and know they live in that place or they're on video or whatever... the person who was threatening them is dead, no longer a threat.
Dunn's situation was unusual because the situation itself gave him a very good reason to drive off, and not only did he not know he'd killed Davis, but he knew there were still friends of his alive in that Durango.
So he's driving off with his fiancee, and he's justified in doing so. But now he's in a somewhat unique situation... there are no flashing lights behind him... he keeps driving... hmm, still no cops...
At the hotel, no cops are showing up. Next morning, still no cops.
Should he have called? Totally. I guarantee you he wishes he had now. But I don't think it's impossible as you imply that amidst all the stress, trauma, self-doubt, shell shock, etc... that he might give in to the temptation to think
"well shit maybe nobody got my license plate number and I know I was justified so, why do I need to throw myself into the lion's jaws here and HOPE that the legal apparatus agrees? I know I did what I had to do, I don't need a pat on the head from a police officer to tell me that."
This is possible. It's also possible he was just telling himself "i'll call soon, let me just take care of ______" like putting off your homework. It's something he was undoubtedly dreading, going to the cops.
Unless you're in that situation you have no fucking clue how you'd react. Your McDonald's situation didn't involve you shooting at the car.