Had a nice discussion the other day that he wasn't really mind-controlled, per se. Everything Arthas did he did of his own volition, he was just given the tools and nudge to slowly radicalize himself due to his own hero complex and the lack of emotional support brought upon him by Uther and Jaina abandoning him to bear the burden of purging Stratholme alone.

There is a bit of both. He did become radicalized the further he went North hunting Mal'Ganis, for sure. He thought he was the source of all this carnage (rightfully so) and he was ready to do anything, including burning his own ships just so there was no turning back for his men.

So everything until Frostmourne was him and only him for sure. But while he did do some very questionable stuff, it could be justified as dire tactics to defeat a great evil at any cost.

After he picks up the sword, though, I do believe he was in great part mind-controlled. Or at the very least, Ner'zhul severely twisted his perception of reality. Maybe he had control over his actions, but his goals completely changed. This would involve killing his father, his own kingdom and most of its citizen to raise them as mindless undead. Something human Arthas would never do.

It's not really anything new either, in fact it's very similar to what corruption does, and the Scourge was originally a tool designed by the Legion. Look at Kael'Thas, who was undoubtedly a good guy before fucking around with Fel magic (and Blizzard's bad writing).
In fact, the very moment Frostmourne is broken, when Arthas is dying, he instantly becomes his old self and seems to regret everything. So there definitely was a big part of mind fuckery involved, but only death could truly release him sadly.