[Polarpew]

I think its clear what happened to Mikasa during episode 7. The realization of "the strong devours the weak" in her soul allowed her physical "limiter" (all humans have it, it prevents self destruction of muscles due to strain), and mental limiter, to be flipped off. That explains how physically strong and mentally in control she is all the time. The whole kill or be killed became her mantra , or her being's raison d'être