The Master always schemes on a grand scale. Let little people satisfy themselves with petty theft and murder; he’s more ambitious, and his intellect so eclipses that of lesser beings that they’re not worthy to grovel at his feet.
No, stealing pretty treasures is nothing when you can take whole worlds, and killing a few people pales beside committing genocide. Most people simply lack the imagination and daring it takes to plan such crimes.
Naturally the Doctor always comes along and spoils his plans, so his crimes never pay, but there’s always next time, and anyway, it’s so much fun!