Merlin caught Zach's tone and he wondered if he hadn't kidded himself believing Zach's initial disgust at the idea that he'd rape someone - who knew what he'd do if he was that angry. Merlin held his tongue, afraid but unwilling to be.
He took Zach's help in getting down only because he didn't want to end up faceplanting himself and potentially being more vulnerable - to say he had trust issues would be a hysterical understatement.
He wanted to tell Zach that he was wrong, that he'd try every possible way around it but he knew that it would happen. He couldn't though, just shook his head and apparated away with an angry crack like the sound of their strange little fake dance of a burgeoning friendship, if that's what it was, breaking.