God Mode: Apocalypse!

He wanted the woman he had feelings for, and she was with the man he had been competing against. When the collapse came the two of them turned on him together and left him for dead. He lived, and a system called Hextech woke up in him. Every monster he killed after that made him permanently stronger and left him a skill he could keep. He used it to stay alive first, then to gather other survivors, and the group put up a settlement they named Dawn City with him at the head of it. From there he learned what had happened to the world. The runes holding its source had been broken and the pieces scattered. He recovered four of them, Precision, Protection, Destruction and Arcane, and the ground around Dawn City came back while the winter held everywhere else. The revenge he came out of that fall carrying got put down somewhere along the way. Between him and the remaining fragments are the Tomorrow Society, which is a cult, and a survivors' base run by people who admit only the sort they think worth keeping.