Quill’s reaction really isn’t surprising, it was foreshadowed in all of the GotG movies. When his mother died, he ran away from his family. When he learned Ego killed Peter’s mother, Peter immediately started shooting. Peter is a passionate man who follows his emotions. I feel like if he didn’t freak out when learning that Thanos killed Gamora, it would have been totally out of character.
I hate how people tear on Quill so much with that. Webs is right. And can you 100% say any other hero would act differently? What if it was Pepper, or Sharon, or Jane, or Christine or... I could go on forever.