How did Itachi obtain mangekyou sharingan?

It was specifically mentioned in the manga that when an uchiha witnesses or suffers from some kind of emotional pain, and when an uchiha who has such strong feelings of love comes to lose them, those feelings turn into an indomitable hatred that consumes them - the anguish and despair that proceeds experiencing such loss, causes their brain to release a special kind of chakra which affects their optic nerves, and it results in the awakening of an eye that reflects their feelings - the mirror eye wheel, or the sharingan.

The more painful experiences they suffer from, the more powerful their eye becomes,and the harder it becomes to reason with them.In other words, the more pain they suffer from, the more powerful their eyes become, meaning emotional pain = sharingan, more pain = mangekyou sharingan and so on.Its not necessary to kill someone, you just have to suffer from the pain of losing someone.

http://www.mangahere.com/manga/naruto/v60/c619/17....

http://www.mangahere.com/manga/naruto/v60/c619/18....

http://www.mangahere.com/manga/naruto/v60/c619/19....

In short, he obtained the mangekyou after suffering from the pain of the loss of his best friend~

No by that time Itachi had the Mangekyo Sharingan and an Uchiha gets the sharingan witnesses a traumatizing even as in losing someone or watching them die which itachi did , they don't get it by actually killing someone since the Uchiha's were revealed to be very emotional so emotions trigger the switch .

http://naruto.wikia.com/wiki/Mangeky%C5%8D_Sharing...