Nana Shinura is Tomura's grandma. Nana wanted her child to stay far away from heroes as possible. When Tomura was born, he was born in a place with no heroes. Something bad happened to him and no heroes were there to help. Tomura was recruited by All For One. Tomura hates All Might because he got Nana's powers when she wanted her children far away from heroes

A summary of my educated guess