Adding to his ego was a lot of power and money that was available through what Russell left in the WTS.
Rutherford I dont think was even on the list of men made by Russell to whom should succeed him stated in his will.
Rutherford's ever present character is similar to which you can see in many men who are positioned in the organization in being bully, over aggressive, arrogant and power driven.
I guess that's what happens to men when they assume they have the power of god within themselves.