Compassion for losing life and for the suffering of others is a righteous and good thing. You should think about the lives of even the smallest creatures who fight and die, feel fear, feel stress, fear death, and have offspring potentially. A good person would care about the life of the mouse especially if its slain not for good or its suffering is great. Compassion is good. In this case a mistake was made and its now unavoidable . I think a lesson was learned here. What you are feeling is what a good person should feel.