Why did I cry so instantly?

Last night when my friends mom dropped me off at my house after swim practice, we saw a possum in the driveway. I put my dog and two cats inside just in case it had rabies or something. Her mom was saying she would just run over it when she backed up but I said no because I didn't want my dad to have to clean up a squished possum and I didn't think we should have to kill it. It was just sitting there snarling at us, we tried throwing some rocks near it to get it to move. But I know they play dead so it didn't even budge. She called her dad and he came over with a 22 to shoot it. All I saw was the headlights of his truck illuminating the end of his gun pointed at the furry little head. I screamed and ran in the house. I immediately was crying. I know I've always had a soft spot for animals but I had no idea I was going to cry over that. Its not like I was attached to it or anything. It was an immediate reaction that seemed more reasonable for seeing a person die, not that possum. Why did that happen to me? It was completely uncontrollable and I'm really confused why I reacted so quickly that way.