Why did Leon Mining leave Zepp?
Prior to 2180 Zepp was a Garrison State… a military force ruled by several higher powers with an iron fist. We tend to call this “Old-world Zepp”.
Until Gabriel challenged the status quo, there were few soldiers who could disobey Zepp’s military system and come away from it alive, especially with the slave-soldier system in place (with bomb collars no less!)
Chances are, Mining got out when he saw the opportunity, though he may be branded a criminal by Zepp at this point, regardless of the past… for his inhumane acts of cruelty towards his own men by performing experiments on them.