Well, I reasoned like this:
When I had to decide what to do with Danny, I offed him right away. He was still a threat in a beartrap, he could have shouted and warned his brother, gotten loose and found a gun etc. He was a threat. I neutralized that threat.
When I had to decide what to do with Andy, he wasn't a threat anymore. He was defenseless. I decided that I don't do senseless murders. It's not like a walker more or less will make any impact on everyday life. What will make an impact on everyday life is the whole group watching me killing someone where it's not required.