Your first question has to do with human nature combined with the lack of an enforcement mechanism. Yes, technically downvotes should be reserved for posts deemed irrelevant to discussion rather than posts contrary to your own point of view. However, it's a natural reaction for many to not even want to entertain an opinion if they don't agree with it. Just look at how people behave in the real world: the difference between a heated argument and civil discourse is that emotions/ego get in the way of a desire to understand one another. Given that there's no way to approve of why people downvote something, people can just downvote for any reason they please. The hidden posts mechanism makes sense only if people are downvoting posts that are irrelevant or nonsensical and don't enhance the discussion in anyway. It's one of life's lessons of maturity to be able to entertain an opinion even if you disagree with it; however with the anonymity of the internet, it's easy to ignore that.
As for your second question, I think it also has to do with the anonymous nature of the internet. It's much easier to talk about someone in an unfiltered way if you do so behind their back. My guess is that before we knew Max was actively reading this subreddit, many people were very unfiltered and blunt in their criticism of Max. I think it was also easier to take liberties in making assumptions of who Max is when we didn't know he was monitoring this subreddit. Once Max made his presence known, it humanized him, it meant that he'd likely read a lot of our comments, and people felt bad for their harsh tone or candidness. I think it's a case where people would have softened the blow had they known he'd be reading their posts. It's like in the real world where it's much harder to criticize someone in a forthright manner if you're face-to-face with them.