What scared him the most about what (little) he remembered was not that he fought his students, not the overwhelming drive, need to kill them. To forsake all that was most precious to him, to rip it apart as savagely as he could.
It was the lust for it all; the bone deep siren call of it.
For every action, there is an equal and opposite reaction.
He'd thought Magnus was his.
Now, between fighting for Apocalypse and seeing glimpses of a dark future in his mind, Charles Xavier's nightmares are not of the humans and their governments coming for his children.
He dreams of destruction from within; he dreams of darkness he had not known was in his heart.
He dreams of fire and death. He dreams of a vampiric touch that lingers too long and one life destroyed because of it, and another forever after steeped in guilt. He dreams of madness and near infinite power.
And yet, his nightmares do not consume him. Throughout the dark visions, there was a constant counter:
The X Men.
His X Men.
He's never been prouder of them, never been so humbled that they hold him in the regard that he's seen both through Apocalypse's dark, too- knowing eyes and his own experience.
They saved him, more than they knew.
- X -
When Kitty dreamt of Danielle Moonstar, we got to see some of the X Men's worst fears; the Professor's was an army coming toward the mansion. What I'm suggesting is that that fear has changed; now he's more afraid of his dream unraveling from inside interference than outside, and that fear is exacerbated by the rage he felt when fighting for Apocalypse. (I'm operating under the assumption that Apocalypse's control of the Horsemen operated at least in part by unleashing their darker sides; that they weren't merely puppets, and that having to see and acknowledge that part of him scares the bejeezus out of good ole Charles.)
Also, when I say "Between fighting for Apocalypse …," I'm not talking chronologically. I'm taking the sum of those experiences- of the fight with the X Men and their allies, and of seeing what he did in Apocalypse's mind- and what the Professor gets out of that is the fear mentioned above.
The following paragraph references Phoenix (rather, Dark Phoenix,) and Rogue permanently absorbing Carol Danvers/ Ms. Marvel.
What saves him from getting (too) depressed over this is that he also saw his X Men fighting the good fight.
Sorry, I don't mean to preach at you all and take away your right to read this and interpret for yourself- or to imply that you can't get it on your own- but I wanted to clarify a little, since I get asked a lot what I mean or to retrace my train of thought. Plus when I reread it it looked a little vague to me.