Why did yandaredevs code suck

I began learning code since I got my own computer a few weeks back and I was wondering, why did yandaredevs code suck so much?

I saw that he didn´t use switch statements and 500 if elses instead, but what else was so bad? Return katana, what is wrong with that. Basically, I´m too new to sit trough a video or read a forum thread joking about his code, but I don´t want to do the same mistakes that he and Piratesoft do without knowing it. Or more bluntly, how do I avoid to become like them?

Thank you for reading!