Board Thread:General Discussion/@comment-26680716-20160501054031/@comment-26887879-20160501162358

Well it's just a fan theory. I guess it comes from the fact you're the real villain and Sans is fighting you instead of other way around. He attacks first and can do betrayal/off guard kill, the thing Player normally does.

"Megalomania" also means mental illness where one thinks they have unlimited power or importance, which reflects main character at that point...