Let me add, you probably have nothing to fear, because mafias have more important things to do than avenge their petty underlings who are pissed about 1 match in a videogame.
In a practical perspective it would make no business sense, and the person who added you is unlikely to have any actual influence in the organization, because successful members of an organization will not have time to waste playing a game like league of legends.
Additionally, they should have absolutely no means of finding you unless you expressly gave them information about yourself such as country and city of origin or other such things.
If you'd like to take steps to protect yourself though, I recommend going through your social media profiles and removing/making private any public posts or information that could lead them to find your summoner name and associate it with your real name.