Never, ever feel guilty or bad for ending an animal's misery. Ever. What you did for the mouse was truly kind and compassionate. And most likely painless, for the most part. Blunt force trauma - IF DONE DECISIVELY AND PROPERLY - is quick and efficient.
Like others have said above, people are so far removed from HOW food ends up on their table, they have lost the ability to comprehend killing. I'm betting if more people HAD to kill that steer or chicken before putting it on their table, there'd be a heck of a lot more vegans out there. Overlook the reactions of your coworkers. They know no better.
We used to own a large farm where we raised turkeys (60,000 when we were full), so I had to learn effective, quick and (hopefully) painless means of dispatching the sick and injured. Believe me when I say that I felt as bad with the very last killing as I did the very first one I did. And I ALWAYS apologized to the turkey before I did it, and asked it to forgive me for what I had to do. I'm sure the turkey didn't understand, but it made ME feel a little better by doing so.
When someone can successfully take the life of another being without guilt, remorse or feeling bad in any other way, that's when it should be questioned. Something's seriously wrong there...
2 Dogs - a Nova Scotia Duck Tolling Retriever, & an Australian Shepherd; 3 Cats; 2 horses, 1 mule; One very spoiled raccoon and the love of my life, Dewey James, and the recently added but much adored, Bobby Lee, brother raccoon to Dewey