Not exactly sure whether it was a manga, manhua, or manhwa, but I remember it being colored, maybe, I could be wrong.
I remember it being a story about a demon king, or lord, or emperor, any of those, who the line of demon kings are always destined to lose to the hero, and he does not want to die. So he goes and finds heroes or adventurers to train them high enough level so he could kill them to get experience and level up. Though he doesn't end up actually killing them from the point I remember leaving off at, and he may never will.
Though he thinks he's weak and will automatically lose to the hero, he is incredibly strong. I vividly remember the start of it being him panicking about that and decides to do this hero training plan, and he ends up saving a couple of people who he is deciding to train, the monster he kills barely gives him any exp. Somehow at some point I remember them being in a desert, but at this point it might just be my mind making stuff up.