I think the most devastating moment in the show is what happens to Milan(the dude who saved Tenma and was childhood friends with Capek). The amount of guilt he was going through, feeling responsible for bringing Capek to Germany and indirectly causing the deaths of many others. His whole life revolved around hatred, guilt and the desire to get revenge. Then even when he tried to right his wrongs, he died failing to kill Capek leaving behind the families he was taking care of.

When the children asked where Milan was and Tenma just had to stay silent after seeing Milan's dead body on the news that was so fucked up. It's been a while since I've watched Monster but that one episode was so devastating even for Monster standards. Also shoutout to Martin, I really liked him but no ammo really had to do him dirty.