In that fight, she was overwhelmed and, in fact, was probably going to die because she was too weak to kill all the wolves. She was also hysterical because she had to kill the king wolf who cast a curse on Subaru, and if left to run, its course would have killed him a few hours later.
So under normal use, she would not have her personality affected, but in this case, she was in a hysterical rage to save Subaru and exhausted almost all of her power on the minions before the boss she needed to kill showed itself. However, why her personality changes when she loses control is not explained, and I can only assume it to be a combination of her already unstable mental state, her being drained of power due to the fighting, and possibly a side effect of her Oni transformation that is normally manageable.
The details are taken from the arc 2 synopses of Subaru's Wiki page