There's actually pretty much the same story between my two brothers. My eldest brother spent a lot of time with a caterpillar and became almost obsessed with it, and then my brother who's slightly my elder entered the scene out of curiosity, asked "what's this?", and instantly squashed the insect with his thumb. It made a huge impact on my animal loving brother. I believe their ages were similar to you and your brother's as well. (They're a few years apart in age.) I was too young to remember the event myself, but my mother tells people the story regularly.

When I was in elem school, I was that kid who was the annoying bug lifeguard in the swimming pool - every damn bug that flew into the pool, I stopped whatever games we were playing so I could save it, even if it was a bee lol.

I was actually a bug lifeguard in high school. There was a wasp on the window and the girls sitting in front of me were afraid. One of them stood up with a sheet of paper in order to do something about it. Sometimes they put the sheet behind the insect and trap it a cup or something in order to free it. But in this case, she just had a sheet and it appeared she wanted to squash it behind it. So I said "what the hell do you think you're doing?", gently picked up the insect with my thumb and index finger, and let it out the window after which it quickly flew away. It's a pretty exciting thing to do. The moment you put it between your fingers, it'll frantically flap its wings, which makes a loud buzzing noise and gives you the impression it's about to sting you, but you've got to simply not let that intimidate you. If you keep your fingers steady, it can't do anything.