John Doom said:
His decision to kill his father is a wasted opportunity, as it should’ve had a greater inpact on his development (also possibly moving the scene earlier in the movie), making him either regret his decision or making him more determined in his actions.
This is something that needs to be in episode VIII. It would no place at the end of TFA. Also you can tell by the look on his face after he kills Han that he’s not entirely pleased.