Why did Johns let Riddick go?

I'm intrigued to understand the motivations behind Johns' decision to release Riddick. Could you elaborate on the factors that may have influenced this seemingly risky move? Was it a strategic play to gain an upper hand, or perhaps a moment of mercy driven by empathy? Alternatively, were there external pressures or constraints that compelled Johns to make this choice? It's crucial to unravel the complexities surrounding this decision and gain insight into the underlying rationale.