If anyone could answer this question.

I can't seem to get my mind off how Erens voice turns so dark and evil like when he goes insane flaming rune mode at the end of the first season in the anime, Is this because he's lost his humanity at this point? Or just because he's really pissed? He sounds the same when he first fights Annie, is it just to let you know he's fucking pissed and gonna fuck shit up? I know I said A question but if you even care I would like to hear from the AoT fans on reddit and who've read the manga (which has no audio but maybe an answer to my question) or just want to talk theories. First post so I apologize if it's retarded in any aspect.