You can rationalize anything.

Just give your character the same motivation as you: To become as powerful as possible. Happens, just look at a famous charter like Raistlin Majere. He was motivated by one thing, power. If you want to power game, just write a story about a guy that wants to power game. (you can even break the 4th wall a little. If your guy figures out he becomes more powerful every-time he kills something, “XP,” then he WOULD go around killing everything if he is only motivated by ‘power.’ I remember one time someone found an exploit in the game to give himself infinite charisma, doing a sequence of ridiculous things. When asked “Why would your guy do that?” He just shrugged an said, “If such an exploit exists, then it means someone in the world could have figured it out, and used it to gain infinite power. My guy could have found it by researching infinite power. OR, my guy knows enough about hows stuff works that he figured it out on his own. Power is power.” )

If you want to play a half angel red-dragon, just write a story where it happens. Hell, I remember reading a story about a group of goblins that was captured and given a drug like ritalin and were trained to be monks as part of an experiment. At the end, they ran out of the ritalin (it was made from a rare plant) but ONE goblin had trained his mind enough that he did not need it anymore and kept the “diamond mind” of a monk. He convinced his trainer to let him go when they went to kill all of the other goblins. He did not want to go back to all of the other crazy goblins.

Hell, I have been able to write a story justifying a Neutral Good fighter that worshiped Vecna.

Its your imagination, the only limits are the ones you give yourself.

tl;dr: “RPG’s for hoarding and grinding or back-story and motivation?” is a false dichotomy.