A lot of people in this thread acting like Arthas’s fall to evil was some kind of organic transition from the acts he committed. It really wasn’t. The organic transition was Arthas going from pure hero to anti-hero. That’s as far as the organic personality transition went.
The rest was accomplished via magic macguffin sword basically stealing his soul and replacing it with a twisted version with zero compassion or empathy for everything he cared about.
Without Frostmorne, you can have Arthas doing some dark acts for the purpose of saving his people - greater good and all that. But to get him to slaughter his own kingdom and kill his father, he had to be tricked into picking up a magic sword that he believed would stop the thing destroying his people at the cost of only bringing a curse down upon himself (a sacrificial act playing upon his nobility), not knowing that the curse would make him a tool of the enemy he was trying to destroy (the Scourge) that would demolish everything he was trying to protect.