I'm so sorry that you had to witness that death and take any part in it. I had a similar mouse trap ("humane" trap but it caught the little guy or gal's tail and it ended up dying :( - my fault, of course, but didn't mean to harm him/her, only to relocate them). It seems impossible to not see that there is stress, pain and suffering there, which I am not mentioning to make you feel worse, and I apologize if I bring it up, but that you sound like an empathetic person in dealing with this little mouse, and the world could learn so much from you and your caring. Thank you for euthanizing your mouse. It was probably one of the hardest things you will have to do. So sorry.