Both are revenge plots. Nam Gi Jun in mercy for none was seeking revenge for his dead brother, going from gang members to gang leaders seeking answers to why is brother is dead and who killed him. Killing each one he come across. In Ae in No mercy was seeking revenge for her sister who was kidnapped and had suffered violence and sexual abuse from bullies. In the mission to find her, she went from person to person (people involved) killing each one. Both got their answers at the end and there was lot of fighting in both.
