Any belief system that requires Joseph Stalin lived to a comfortable old age, dodged any ultimate accountability, and got away scot free with a lifetime of unprecedented inhumanity and cruelty, has no standing to complain about the concept of justice in other belief systems.
Incorrect. When discussing theology one can easily (even while lacking a belief in God) critique the CONSTRUCT in the theology.
Let us say that the argument is "The All-Powerful, All-loving God loves us and cares for us" (which is a pretty standard argument) but this same God I am told loves and cares for us also commanded at least one if not many genocides, destroyed all live on the planet except for a small group, allows for horrible cancers to consume innocent children every day across the globe, etc etc.
As an atheist I can say "That doesn't sound logical or reasonable. Further, it allows me to assume that the construct the other person is calling "God" can be rejected since it makes no sense theologically.
Does that make sense?
If I am asked "Do you believe in God?" I can say "I fail to believe in God because God is either well defined and logically inconsistent or he is so undefined as to be effectively meaningless so I feel bolstered in my failure to believe in this thing."
So OF COURSE I have standing!