This has nothing to do with whatever Mark said in his live stream. I don’t know what happened. I didn’t watch it. Just in general…
You can’t tell a creator what to do with their character. That’s not how any of this works. That’s not how it ever has worked. Headcanons and AUs are there so you can mess with a character someone else has made, but the truth is, as soon as you touch that character, you’ve made someone entirely new because no matter who you are, you will never be able to write that character the same way the creator will. Even if you aren’t putting your own headcanons into that character.
Maybe it’s because this is YouTube and it feels less professional than published books or TV shows, maybe fandom on other platforms do this too. I don’t know. I haven’t spent enough time anywhere else to speak for it, but creators don’t owe you anything. Damien isn’t yours. Neither is Warfstache or Darkiplier or The Host. Anti. He’s kinda yours. Being a creator- using other peoples’ characters- means creating things that are wrong. It’s just part of the package deal. In any fandom. By all means keep doing it, but what you make is probably not 100% accurate. And it definitely isn’t if you’re putting things into that character that were never specified. That’s just how it works.
Go ahead and write and draw a character however you want. Just know that everything is technically “ooc” Because the only person who can make a character “in character” is the creator. And they don’t owe you anything.
You can have all the headcanons you want, but they’re not canon. Canon is whatever the creator wants it to be.
If you don’t like the canon, you can totally write fix-it fic or come up with an alternate timeline where X Thing You Hated didn’t happen.
As long as you respect the creators’ work, it’s all good. But don’t fight them on what “should” be canon. That’s a fight you will lose.