why did Viktor Chemmel assumed the leadership of the gang of thieves?

I believe the leader Arthur Berg moves away making Victor, who was second in command, leader.

He rose because he had no opposition. He understood the concept, "United we stand; divided we fall." The village thieves either didn't recognize the difference between Berg and Chemmel - or didn't care. Chemmel slid in easily because of the apathy. Where Berg's motives were rascal survival, Chemmel's were inhumane sport.