I really like Melon as an antagonist. He's psycho, and kind of a dick, but he knows his shit and has charisma, not to mention he also comes across as even more threatening than Riz because of how unpredictable he is.
Also despite everything, in his own wierd way, even from the start I think he genuinely liked Legosi.
That being said, for a series that usually seems at least (barring the odd moment) somewhat grounded in a sense of realism, these last couple of chapters with the random quiz of death led by tiny executioners seemed to come out of nowhere, and felt kind of jarring tone wise.