Did I ruin my best friend's life?

You say you're 27 in the story, but when I checked your profile you say you're 22 and a half. I'm not sure if this story was meant to be true or to shock someone. If it is true I think you do have issue. Just because it's annoying you could have ask your friend to moved the dog somewhere else. You don't kill someone's pet, It's like killing a human but instead it's an animal. You should be truly ashamed of yourself for killing a dog. It doesn't matter if she couldn't control her dog. You either just try to ignore or don't go to her house at all or she can go to your house.