Meisner is the leader of Hadrian's Wall, an organization that is aware of Wesen and Grimm. The primary function of Hadrian's Wall is to neutralize dangerous Wesen. In this instance, however, Hank was abducted with the objective of leading Eve away from HW. This information was revealed after Black Claw had already initiated an attack on HW without their presence.
You were with Black Claw, recruited by them at a young age because Conrad Bonaparte was your uncle who got custody over you after your parents died. So you were there too when the chaos broke loose, Black Claw agents attacking Hadrian's Wall and killing everyone there. Yet, you felt it was not right, as if you didn't want to kill all these people and eventually all humans so that Wesen rule. So when you find yourself holding your gun at Meisner, you pause, and instead of pulling the trigger, you move away from the room and go to another place where they were fighting.
Eventually, the majority of people had already passed away. There were a few Black Claw agents left, as well as Martin Meisner, who did not have a difficult time killing your "colleagues" from Black Claw.
When he eliminated the others, he was standing in front of you, but he hadn't yet initiated killing you. Remembering the mercy you gave him earlier.
''You did not kill me, why?'' He asks, looking at you carefully.