Well, sometimes when we do something without much thinking, we start to think what would happen if we didnt do that. I think that that is what you are doing now. "What would happen if i wasnt with aunt", "what would happen if i felt this way" or "that way" etc. But thing is the thing you did and cant explain why you did it, you did it from your subconscience . Its what you would want to do, without thinking, you did it automatically, since it is situation that is quick so there is no time to think.
We all have some sort of dark thoughts, like you wishing for puppy to die. But most of the times, they arent rational, they just come from some dark place in our minds when we are feeling dark, or when something stressful happens etc.
You aint a bad person Penguin, you saved a life today. YOu may think you wished for that life to die, but you saved it. What i think is bothering you is that you think that your action was against your wishes. that you did something that isnt what you would usually do. But trust me, that person who saved the dog, that is you, just cut off from all of the thinking.
You are a hero.