I was looking at the Professor X page on Wikipedia as I prepared to write this, and noticed the information on his "dark side." Let me tell you what, his "nice side" isn't always so pleasant either. (The clip I want you to look at is from about 1:11 to 2:22.)
So what we have here is the good Professor probing someone's mind without consent. Never mind that it's the royally screwed up Sabretooth whose mind he's probing. Never mind that he intends to help.
This is distinctly unethical. Against Sabretooth's will, he is entering his mind and trying to find things that Sabretooth has sealed away. (Also, never mind that it turned out Sabretooth was actually working for Magneto and was intended to take the place over.) The ethics of characters with telepathy in fiction have nearly always been kinda... unpleasant, and this was in a kid's cartoon.
And then, later in the same episode, he chases away Magneto, formerly his friend, by making him think of bad, painful thoughts. Freezing a bunch of people in their tracks is one thing. Making them relive their nightmares is another. (Of course, it was self defense-Magneto was intending to crush him to death. But that's not the point-this kind of act is often called mindrape or worse for good reason.)
And then there was the time in Secret Wars where he wiped Spider-Man's mind to keep him from relating that the X-Men were about to betray the Avengers faction and join Magneto. He commented later that "I may have overstepped my boundaries." Gee, really?