Humans do evil things to each other. We pretend other and hope to change that. We largely fail.

The root of most of the world's problems are simple.

EITHER:
There's a pile of $$ over there and I want it.

OR:
I made or have a pile of $$. I want more and no one else can have it.

@WarnerCrocker
I would even say, it's not money as the root cause, but power. In a society without money, certain people would still seek power with other means. I am rather pessimistic that us primates will ever overcome that desire or will find a way to mitigate the negative effects.
@nibbs IMHO, $$ is the conduit to power, and now substitutes for it. Even when actual power doesn't really exist.