YEAH. You start to go berserk!
So you've entered a frenzy! You're now really angry and you will also do better in combat for some time. You're stronger, you have better senses and you're harder to feint! Being in a frenzy is especially useful when your character uses clubs or fists. Sadly, you won't spend a very long time in this state unless you keep on making those fatalities or using some other method.
Entering a frenzy
Frenzy is something you normally enter by performing a fatality. Fatalities are finishing moves that you will do sometimes by using the aggressive or the berserk fighting style. You can also achieve a frenzy by using the Bleeder mutation's command or by using on someone as a Hideous Freak.