... regardless of what you or I think about stained at birth, there are people who buy into it, and that might be the reason for the seeming contradiction you pose in your OP.
This topic would likely never come up in a minarchist/anarchist debate because the people who think that people are inherently bad just accept it subconsciously without stating it as an explicit premise. Therefore, it remains a HIDDEN premise of theirs, which leads to what you perceive as their inconsistent conclusion. Their conclusion is not inconsistent when their hidden premise is revealed.
It is usually those hidden premises that are the true reasons we disagree on our conclusions.
Just find out how many pro-minarchy/anti-anarchy people (a) are theists and (b) believe in stained at birth.
Their answers will prove my hypothesis either right or wrong.