I don't think he went from being calm to crazy, but more like just losing self-control. Like, it was revealed in the end that he had been manipulating Locus the whole time that they knew each other after the Great War, and that he really was just a sadistic douchebag the whole time. Felix knew that he needed Locus to have his back if Felix was going to survive out there. However, when Locus started questioning things, and when thier mission began falling apart, Felix realized he was losing control of the situation, and therefore began losing control of himself. For example, telling Locus "Fuck the mission! For once in your life think of something other than following orders!" (or something like that) was a really bad idea, considering that Felix had been building Locus up to be this mindless drone who just follows his orders, no matter how brutal they may be. Someone as manipulative and sadistic as Felix would obviously put on a different persona and act like a stern and calm person in a military scenario, like he did in Seasons 11 and 12, and in the flashback in Season 13. But when Felix had lost his control over Locus and the war, his real personality began to show.
So, I guess in a sense he did actually go off the deep end, but the deep end was just his fake personality where he portrayed himself as a calm, cunning, merciless individual. I mean, he did give the "performance of a fucking lifetime" during Season 11 and the first half of 12, so that clearly wasn't his real personailty.