Do you realize that most of the issues he had was her the reason? The corrupt country, the pedo hero, the crazy lesbo hero, who do you think was behind all of them pulling the strings? Instead he wipes her memory and use her to do... things that are actually pretty minor compared with the main issue.
Its like saying that the Count of Montecristo brainwashes Fernand Mondego to help him to get the others instead of making him suffer because it would make things easier at the start. He doesnt need her moreso when she can have the op beast girl AND the Sword Saint that is on par and even stronger than some heros (and the story is just starting im sure more will come later, all females of course so he can "level them up") maybe it would take longer, but revenge is always a dish served cold
Its kinda stupid to let off the hook the main villain just to get some minor ones instead
On that sense Nidome no Yuusha is more coherent plot wise, he hates the princess so bad that he wont even feel lust for her and focus on make her life as miserable as possible, probably leaving her for the last trying to prolong her suffering as long as he can