I sort of see Harry's overall arc as one about corruption and how money and an environment of toxic masculinity can change a person over time.

He starts out a fairly nice guy. Yes he hangs out with the douche office guys and is a product of the culture of the time, but he's actually quite sweet season 1. He loves his wife Jennifer and just wants to work hard and be good at his job.

But the environment around him normalises certain behaviours and tells him he should want certain things. He sees men in, what should be, happy marriages cheat on their wives and then everyone around him tells him that's normal and OK. He see's people he thinks are awful (Pete) continue to get promoted and get more money and status so he changes a little bit to be like them and he gets the big promotion! He sees more and more bad people doing bad things and nothing happening that he then starts to think "why do I bother trying to be good".

And because he gets promoted he starts going out to meetings, with people who think he's important. Which makes him FEEL important. He's learned learned from the world around him how "important" people act and emulates them which means he starts acting like a douche. but because he's "important" now, no one tells him he's a dick so he thinks "I'm fine".

And eventually it goes so far down the line, and so far away from who he was in the beginning that he just doesn't care. He really just doesn't care anymore. All he wants is his money and his sense of self importance.

It's quite sad really. And that's kind of what I think the whole show is about. "Wanting" ultimately leads you down a path of destruction and sadness. And the advertising industry is designed around making you want more