so the champions of the light would come to him, so he could raise them.Number one holding back the Scourge is a canon game lore. If you disagree with it you can talk to the developers that made it.
read the book, the sword made him mostly a psychopath, it didnt take his free will. I hate having to explain this, but it gives me a great idea for a fisher price lore book.He was doing that all the time.
Number three the Real Arthas came back when the sword was shattered meaning that he didn't had control of his actions