Not everyone is as honest as Destiny (the streamer) in this video. He frankly says that he isn't sure if anything is really wrong with doing even the most vile acts he can think of. Most people who appeal to subjective morality aren't as straightforward about this.
In his case, the rejection of morality seems to derive from a rejection of God. It's a bleak and hopeless worldview which, thankfully, is not true.
My website https://BibleThinker.org