Omega Red's ending shows him having killed Ryu, where a deeply distressed Ken asks him "How many lives have you ended?". Omega Red merely responds "Do you count the number of breaths you've taken?". 

This is a reference to a similar scene in the manga series JoJo's Bizarre Adventure, when Willian Anthonio Zeppeli asks a similar question towards the vampire Dio Brando. William asks Dio Brando "How many lives have you sucken away to heal those wounds?", only to be answered cynically with "How many breads have you eaten in your life?".