Demon In Attack On Titan

A boy reincarnated into the world of aot, all alone, with no friends or family, with only a system that promises him unmatchable power. What will be the fate of this boy? Will he be the savior the world needed or the very thing that caused the world to end? First book ever, so please don't judge it like it was written by a pro. If you want the story to be better you can assist me with that, just give me advice and let me know my mistakes.