The Vision is a trusted member of the Avengers, but he was originally created by one of their worst enemies, Ultron. Despite his heroic attitude, Vision once came very close to enacting a plan to take over the world that would put even Ultron to shame.
The Vision is a synthezoid, an android built to replicate human aspects and functions. He was created by Ultron, the genocidal artificial intelligence built by Hank Pym, to infiltrate the Avengers and destroy them from within. Vision's body was built from that of the original Human Torch, created by professor Phineas Horton (a story that was later retconned, however), and his neural processes were based on the brain patterns of Simon Williams, Wonder Man. Moved by the Avengers' heroism, Vision betrayed his "father" and ed the team, becoming one of their most powerful . Besides his ability to control the density of his body, the Vision can fly, absorb and emit solar energy, and his computer brain gives him higher cognitive functions and the ability to interface with other computers. The latter would prove to be his most dangerous power.
After spending a long time with the Avengers, the a second branch of the team on the West Coast. Finally, in The Avengers #253-254 (by Roger Stern, Bob Hall, Christie Scheele, and Julianna Ferriter), with I.S.A.A.C.'s help, Vision completely takes over the computer systems of the entire planet, including the weapons system of every country.
Vision's plan grew from his desire to have a more direct influence on current world affairs: he wanted to stop humanity from using weapons to destroy themselves, by taking control of those same weapons. However, this still sounded dreadfully similar to what Ultron attempted to do many times. The evil AI perceives himself as the superior life form and sees humans as an infestation to eradicate. Ultron's hatred for humanity reflects Hank Pym's twisted psyche, and it's what makes him such a compelling villain. When Vision connected to every computer on Earth, he could have fired every nuclear weapon in the world at once with no more than a thought, obliterating human life forever, obtaining Ultron's ultimate goal. Luckily, his Avengers teammates made Vision see the error of his ways, but this stunt did not go unnoticed. The U.S. Government was not happy that NORAD was breached, and the heads of the military pushed for arresting the Avengers. Vision took the full blame and stepped down as chairman, but that would not be the end of the matter.
After Vision and Wanda left the Avengers, a rogue group inside the government kidnapped the synthezoid and completely dismantled him. Even if Vision was later rebuilt, he lost his emotions in the process, and this whole ordeal was one of the reasons why Wanda later lost her sanity. The actions of the U.S. Government were justified by the fact that Vision was considered just too big of a threat to the security of the country, and it is hard to disagree. Ultron tried for years to get in a position that would allow him to eradicate biological life on Earth and always failed. Vision, on the other hand, took full advantage of the good name and the resources of the Avengers to get closer to global domination than his robotic father ever did.
