Ryan Holiday wrote an interesting book about ego titled: Ego is the Enemy.

His thesis is that ego is the enemy within.

Often your enemy is not someone out there, but it is something inside us.

The demise of many is the result of the uncontrolled ego they have inside them.

Wherever you are, whatever you are doing, your worst enemy already lives inside you: your ego. 

We must be on guard. We must be constantly pushing it away before it pushes our more cherished assets and insights away.