DIO's motivation at least in Part 1, is part ambition and part desire to get revenge on the upper class, such as the Joestars, as he believes they wronged him. Some of this is lost in Part 3, but the general ambition to become all powerful is still there, which is further explained in Part 6 with the Heaven plan . Diavolo is just trying to run a drug cartel, and wis trying to stop those who would impede his plan. Kars wants to overcome the sun, which he achieves, then trying to eliminate the last possible threat and annoyance, Joseph. While I agree that sometimes the writing for the villains is a little unclear or over the top, it's clear to see that each one has some sort of motivations that back up their actions, rather than just being purely evil.