In the past, my pets never received a sedative before euthanization. But, I have to say, that they never flinched with a fight or flight reaction, just slowly relaxed in my arms until it was over. Now, last year I had to put down my girl and the vet actually asked us if we wanted to give her a sedative first. I was never even aware of that, but since he asked, we said yes. She got the sedative, and 2 euthanazia shots afterwards, since there were so many strong drugs in her system from years of epilepsy, the shot didn't take effect initially.
You need to just let this go, he's resting peacefully now. You can't judge the extent of any pain at the time, who knows, it may have just been another feeling that caused the reaction. Like Goldfinch said, you did your best for your baby, and you should not be having any guilt feelings about this now. It's not always healthy to overthink these things.
In my case, I talked with my husband afterwards, and we both agree that now that we know about the sedative shot, we will insist on it in the future. Your little one is at peace now at the bridge, you should feel the same. (((hugs)))
"We can judge the heart of a man by his treatment of animals." ~Immanual Kant~