Pre-warning: It sounds like you want to make sense of this. To do that we have to think about it in some detail for just a bit. I'm going to use some descriptive language that is not meant to shock or offend but could be upsetting none the less.
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Let me address your regrets...
I feel so terrible that we couldn't do more. I regretted that I didn't hold his hand as he died.
Based on your description of the accident, it sounds like he was buckled in. Roll overs are very good at throwing people who aren't buckled, especially when the door is compromised.
So he was rattled side to side with extreme force. Bear with me, this might end up being comforting in a sense... Within a fraction of a second of the man realizing something is wrong, the truck has gone sideways and rolled. Side curtain airbags are a thing, but this doesn't sound like the kind of truck with them and in any event, they do not continue their protection beyond the first impact. So in less than a second he's gone from zipping along (too fast but that's how some people do it) to having his head bonk the door window and or ceiling of the car. In that blink, his awareness was shut off like a switch.
There is much to regret over any senseless loss of life, but offering him comfort in this moment would not have made much difference to him, he just wasn't there.
As for whether or not you could have done more. No. The crash broke the man's ribs and punctured his lungs. If a fully equipped ambulance had been on the scene when it happened they may have been able to stabilize him, but the EMT being 15 minutes away essentially made this outcome inevitable. Even if they had got there immediately, I'd be generous to put his odds of pulling through at 1 in 20.
As for the guy pulling him from the truck? They definitely tell you not to move an injured person in most circumstances. This would definitely have been one of those circumstances. The main reason is that if the man had a broken neck, moving him could have caused damage to his spinal column. Moving the man did him no favors, but it's not the reason he died.
Sounds grim and horrible. I'm sorry for that. The take away is that he didn't suffer, and anything you think you might have done differently almost certainly wouldn't have helped.
Here's the good news... You learned that you would do
something in the event of a tragic accident. Many people don't. They gawk, get their phones out and call maybe. Maybe they record the scene on video for the news to play later. You did something. In this particular instance, it would never have been enough, next time it might be the crucial decision that saves a life. It's never a source of regret to try your best and fail.
Here's a bit of cautionary tale... The fact that you're thinking about this, this strongly, after the fact is an indicator of traumatic stress. What you went through really isn't something we're programed to handle, and if you think it might be a problem then definitely err on the side of talking to a professional about it. Doesn't have to be a big deal, see a brain wrangler once a week for 2 or 3 weeks, make some time with a clergy, really any occasion where you can talk about what you went through in a normal way. Not always an easy thing to do with friends or family but sometimes that can be all you need.