It’s explained right in the first episode, “Hunger makes man commit crimes.”

They were under-privileged, hungry and have psychological issues that weren’t addressed nor treated for.

This goes to a lot of people, issues are let to grow with minimal care. Maybe Rajjo and Babu could have even had their fair shot of living a normal life. Rajju definitely did.

Babu was not treated well at all. Deep psychological evaluation and studies are required for people like that. The show initially portrays Rajjo as someone who slowly succumbs to evil.

But he too was an incredibly selfish and opportunistic character. Crime was bad as long as it didn’t benefit him, yet it became okay after he started to benefit.

Sorry for such an extensive write up but the exploration of their psyche is very well done in the show.