Towards the end of the movie, Kung Fu Hustle, the beast leaves his cell at the behest of the axe gang. When being briefed by the gang, the beast says he could have left his cell many years ago, but didn't, because he didn't see a worthy adversary for himself in the world of Kung Fu.
Now, we might assume that the beast leaves his cell to fight the pig sty alley couple, but there's a catch - he knew who they were, and that they were out there. This is evident when he calls them and himself the "three true masters of Kung Fu".
The beast didn't leave his cell to fight the landlord and landlady from pig sty alley, but because he found a worthy adversary, and possible superior to himself, the main character, Sing.
Notice that when Sing opens the door of the beasts cell, the beast just stares at him. He's clearly intrigued, and possibly even sensed the Chi Fow flowing through Sing. The reason he followed him out wasn't because he wanted to fight a pair of kung Fu masters, but because he sensed potential, unprecedented potential.
It's clear that the beast has a superior knowledge of kung Fu to anyone else in the world. It is plausible that he can sense potential in someone without actually fighting them. Maybe he beat the hell out of Sing knowing that the beating would unleash the Chi Fow.
Perhaps the emotion he showed getting down to his knees and acknowledging Sing as his master was actually out of contentment that he was right all along.
I know this sounds crazy, but it is a theory about a well...crazy film.