He is fed a lie by a villain that the rest of the class killed his brother (who of course, he doesn't even remember and therefore is not a motivating factor, also not true), which makes him even more hostile to everyone else, so far as becoming a spy for the corrupt military commandant. A role which sees him provide such valuble insight as, "I HATE CLASS ZERO, THEY ARE JUST FOLLOWING YOUR ORDERS! I CAN'T BELIEVE THEY KILLED MY BROTHER. WHAT WAS HIS NAME? WOW. I HOPE MY SPYING IS USEFUL TO YOU AND THEY ALL DIE BECAUSE I AM THE WORST"
He becomes so obsessed with the idea of Rem remembering him that he decided to become a L'Cie (yep, from FFXIII, here nobody forgets a L'Cie because how could you forget about Lightning?) just so Rem won't forget him when he dies. Oh, but he becomes a L'Cie WORKING FOR THE FUCKING BAD GUY'S SIDE BECAUSE DEAR GOD HE IS THE FUCKING WORST. So he is not even playable during the two largest scale missions, not that you'd want to play as him anyway, but way to contribute. Great playable character.
As you enter the point of no return, Machina decides he's going to just leave for good. Shortly afterwards, Rem's TERMINAL ILLNESS nearly finishes her off, but in desperation she accepts an offer to become L'Cie for the good guys. So guess what? Wearing a mask, that while hiding his idendity but in no way obscuring his vision, Machina then proceeds to BRUTALLY FUCKING MURDER REM. (Rem not wearing a mask by the way. Rem still wearing her uniform. Guess she should have worn a BIGGER FUCKING CAPE?). He then takes off his mask and cries about it as she bleeds to death, as if he hadn't just done it with his own two hands and isn't the biggest piece of shit in video games history.
Oh and then the whole class, who he was a dick to the entire time, sacrifice themselves in order to create a new world free of War and Machina and Rem somehow get resurrected and Machina is hailed as King of the new world, marries poor Rem and dies peacefully of old age in some truly unfathomable bullshit writing. FUCK.