I'll try to recap for casual fans. The Cyclops character arc has been gradual for over a decade in the making though. It's kind of a "heavy is the head who wears the crown"-type thing based on being thrust into a leadership position based on a series of tragic events and it makes sense and hasn't been forced imo.
His personality became a little darker and ice-cold under Grant Morrison's run on New X-Men back in 2000. He explained it saying that during a wack plot from the late 90's where he ended up merged with Apocalypse, it was like being possessed by a demon and he Apocalypse showed him how much his heroism is based on delusion, and it made him cynical and withdrawn. He was on rocky terms with Jean Grey and was having a psychic affair with Emma Frost. Then Jean Grey got killed in front of him by Xorn, the Magneto clone, so that fukked him up.
Then his brother that he never knew he had, Vulcan, showed up and killed Banshee in front of him. He then discovered that Professor X lied to him and all the X-Men for years, and who we thought was the 2nd team of X-Men was really the 3rd team. Xavier sent a 2nd team of X-Men to rescue the 1st team from Krakoa, which was led by Vulcan. Cyclops watched Krakoa killed them all in front of him back then and ran away, and Xavier wiped the entire memory including the existence of the 2nd team and his brother from his memory. Cyclops discovered what a scumbag Xavier is and told him he needs to step aside and he's leading the X-Men and running the school and told Xavier he wasn't even a mutant anymore (he wasn't at the time after Decimation). His other brother Havok, ended up stranded in space as a prisoner of Vulcan, after Xavier sent him on mission after Vulcan so he blamed Xavier for that too, understandably.
After Decimation (the Scarlet Witch making 90% of mutants lose their powers and making it so that no new mutants were born), there was less than 200 mutants left in the world and they were on the verge of Extinction. His wife was dead and he was betrayed by his mentor and father figure in his eyes, and he felt Xavier's dream of coexistence with humans wasn't relevant anymore because there only less than 200 of them left and they were just fighting for survival at this point. He knew humans would try to wipe out the few humans left, so made a pre-emptive strike team, X-Force, led by Wolverine and ordered them to start killing their enemies first.
Norman Osborn became the corrupt director of what used to be S.H.I.E.L.D. and decided to send his Dark Avengers after the X-Men, who led by Cyclops had moved to the bay area by now. The mutants fought them off with the help of Namor and Magneto, who now had joined the X-Men (Magneto kneeled down to Cyclops and told him that he united all the mutants, which he and Xavier never could do) and moved all the remaining mutants to Magneto's old Asteroid M, which was now off the coast of San Francisco, and he called it Utopia.
His previous hunch about creating X-Force to preemptively strike against humans who were plotting their extinction turned out to be true when Bastion united the Purifiers, the Sapien League, the Reavers, the Friends of Humanity, and every extremist mutant-hating group and went ham at all the mutants and Utopia and tried to take them out with Nimrod Sentinels and shyt. Cyclops led them to fighting off another attempt to wipe out the remaining mutants. Beast's lame ass found out about X-Force and confronted him in the middle of the fight and he was like "Yeah we're killing people. I regret nothing.
"
After hit was done, Captain America, who is now the top cop in the nation, gave him the medal of freedom for defending San Francisco and he threw it in the water and said he doesn't respect human recognition. He had Xavier and Magneto as his two advisors, but he seemed to be gravitating more toward Magneto's views.
He found out that the Mayor of San Francisco had authorized the military to blow up Utopia when they were attacked by Juggernaut who was possessed by a Asgardian force that made him even more unstoppable, so him and Magneto walked in her office and threatened to throw her out of the window.
Wolverine turned on him over some newly-found moral objection over using kids as soldiers in the battlefield and moved back to New York and about half the X-Men joined him. Cyclops put together the most powerful X-Men team ever and called them The Extinction Team and started sending threatening letters to humanity saying that they're heroes and going to protect the world, but won't tolerate any more attacks against mutants.
The Phoenix force came back to Earth and was seeking Hope, "the mutant messiah" on Utopia. Captain America and the Avengers along with c00nverine decided they had to take Hope into custody (Wolverine wanted to kill her), but Cyclops said no because he thought the Phoenix was coming to her to restore mutants being born. The Avengers fought the X-Men and Cyclops ended up being the Dark Phoenix. Xavier fought him and was in his mind trying to take him out. Cyclops wasn't in control of himself and lashed out and killed Xavier.
Turns out he was right and the Phoenix did come to re-spark the mutant gene and new mutants were being born again.
Afterward, Captain America put Cyclops in prison. Magneto, Magik, and Danger broke him out. Since then, he's been an outlaw fugitive with Magneto, Emma Frost, and Magik, living underground sending messages to humanity talking about leading a mutant revolution and they're still taking in young mutants and training them.
this is so much better than that lame ass 
cyclops from the 90s xmen cartoon
moment for me.
Cyke is cold.
Once Scott got ahold of that Dark Phoenix force though... 
