Intelligence exists, but there is no freedom of thought. This is what distinguishes a tool from a person, which is why it cannot be said that Arthas went anywhere on his own; he didn't choose the goal, and he couldn't refuse it.
Oh, of course, that's how it is. But since the blade imposed its will on him, what did Arthas become? Wasn't he the embodiment of the blade's will?
Except the ones we've made