Joe Chill

Joe Chill was a common crook who ended up killing the most prominent citizens of Gotham, Thomas and Martha Wayne, leaving their son alive to become The Batman.
Feeling profound guilt for not having saved his charges, in spite of his exceptional combat background, Alfred Pennyworth tracked down and killed Chill.
Years later, he would tearfully retain this information from Bruce, knowing that, symbolically, the idea that any of the criminals he fought could have been the one to kill his parents was a critical aspect of what made Batman Batman.
However, he penned a letter to Bruce, which he read after Alfred's passing.