Why take over the world?

My bad guy needs a motive for taking over the world, but what? In the story, the main characters is trying to avenge the one responsible for sending spells which caused the deaths of many, many people. It is found that each time the gone-bad-wizard, the bad guy, killed a victim with his curse, he gained all of the power that person had owned. Over time, this dude just keeps getting more and more powerful, so, essentially, he was taking over the world. But why is he doing this? Anyone have ideas for a motive?

Maybe he thinks he can make it better. "No, you're doing it wrong, let me." There's some major issue on the planet (say, overpopulation or disease or global warming or magical bad things) that no one is really equipped to deal with or people are dragging their feet arguing, rather than fixing it. He just wants to resolve the problem, but first he needs to bend the whole world to his will.

because I am the only one who sees clearly what the problems are and how to cure them. I can get it done directly much faster than through a system plaqued by political wrangling. see a problem, solve a problem, move on to the next problem.

everyone I kill is a hindrance to the cause alive, and a contributor to the cause by his death.

villains trying to take over the world are so cliché. you can use it by presenting it in a new way like - the protagonist investigating why the baddy is killing people and gaining their power

Maybe absorbing power is addictive?

Simply becoming more powerful is certainly addictive.

What's the old saying? Power corrupts, absolute power is even more fun!

To breed Heros. Kind of an extension of onesecondglance's idea. At a certain point, he has the power of god and killing and absorbing more people won't noticably effect his power, but what if the power absorbtion process causes an intense jolt of pleasure? He knows that any time there's a dark lord of the earth, hero's rise to the occasion and attack the cruel overlords and the worse the regime the more powerful the heros, the more powerful the pleasure jolt that the villan gets. It's also possible that over time the power fades without fresh blood, so it's in his interest to keep the supply up.

I agree that once he has started, the draw of more and more power is like an addiction. If great and new power is at his fingertips, he is going to grab it. But the trouble is, I think you need a good reason for him to start, some breaking point that maybe even is an accident that he does this for the first time. Something like a fight with a childhood rival over a girl when he is a young man causes him to lose control with rage and maybe he does this spell thing for the first time. Then, though maybe a little disgusted with himself at first for murder, the thrill and the power, and the rush of having gotten away with it, eventually pulls him in and he does it again. After that, he is twisted and evil and can just keep going.

So basically, i think he needs a trigger event. (thief kills his lover so he kills the thief was another idea i had)
Then he needs to realize he likes it.
Then maybe he gives himself a justification and convinces himself he believs it, like (at the begining) he only kills those who he believes deserve it. People who have commited crimes. And that with this power from these deviants, he can do great things.
After this, feel free to turn him into your evil, monstrous murderer who, by this point, has become just that because power corrupts.
Hope this helps! :)

I just realised that the whole idea of a person becoming addicted to stealing others' powers is basically Sylar from Heroes.