Technically it would have been in cold blood, since he was no longer an immediate threat to Glenn. They made a point to show Glenn's inner turmoil at that moment, I think for the reason I stated earlier in this thread. He is trying to hold on to his humanity. Killing Nicholas in that moment would have changed Glenn forever - he's never killed anyone in cold blood before, it was always either to protect himself or his family. While it would have been satisfying to see Glenn get revenge, I suspect the decision to stay his hand will be a boon for Rick's group in the next season.