As the oceans warm, the water vapor in the air increases. Hurricanes are fueled by water vapor in the air, and the water temperature of the oceans sets the intensity they can potentially become--the higher the water temperature, the higher the possible intensity.
So in general in a world with warmer oceans, hurricanes can on average reach greater intensity. There are several other things related to global warming that can make them worse--with more water vapor the rain intensity grows greater. Environmental wind shear may also decrease in a warmer world, because that's determined by horizontal temperature gradients through something called the thermal wind equation.
In any particular storm, though, it's almost impossible to say exactly how global warming might have affected it.