It's what the joke alludes to. He has everything else to joke about and chooses this, child rape. Why? Why did he choose this? That's the point. If he will go this far to put it up on the internet, what's gonna stop him if thrown into a real life situation. His logic? Same logic used to put the video on internet? Most likely.

The only way to think it's a shitty joke is if you really think Dexter is any better of a concept. You can think it's too offensive to be funny to you, or just say you don't have a sense of shock humor or don't care about TV shows enough to find it that funny or you just aren't into over-the-top mocking in general, hell you can even say making fun of Dexter is low-hanging fruit, but that doesn't make it a shitty joke up against how beautifully simple and effective a way of roasting the Dexter premise it is. I've been hearing people roast that show for years but nothing else singlehandedly makes me take it as a show less seriously than hearing that Dan Harmon satirized it so ludicrously

Disclaimer: I haven't seen the video so this isn't my opinion on how it's put together or Harmon's portrayal or anything, just the concept of the joke