What does 'genuine good intentions' even mean? Whose intentions? His, yours? Mine, or the ones from the guy next door? Who decides what good intentions are?
Don't get me wrong, I think the circumstances that lead to a dictator becoming a dictator make it very unlikely, but to call it impossible just seems crazy to me.
I assume the benevolent dictator would be someone, who allows different opinion, and who allows his policy to be changed by his people. And if they want to be governed by someone else, he would step down, have his own power limited or stripped. That wouldn't be a dictator then.
And you'd still have to deal with his administration which has it's own momentum. The 'benevolent dictator' could simply be replaced (killed) by his own clique with someone more in line with their interests.