I don't think that Faith loves violence as much as she enjoys the contest, She revels in her strength and takes pride in her skill, certainly, but more like a champion athlete than anything more sinister. Her enthusiasm makes her take more chances than Buffy normally would, but that's just more her love of "the game," as Spike would put it than any real desire to inflict pain just for any sadistic pleasure.
Faith is like the gifted athlete who sees the competition as a way of proving that she has genuine worth, in answer to those who, all of her life, have taken great pleasure in putting her down.
Neither girl enjoys the aspect of cruelty and the necessity of inflicting pain on others, they simply recognize the fact that inflicting pain and death iare aspects of their calling. Faith was no more cruel in her torture of Wes than Buffy was when she put the crucifix in the vampire's mouth in Season 2. Both acts were means to specific ends. Buffy knew that Giles was in danger, time was of the essence, and action had to be taken immediately. Faith, on the other hand, knew that Angel would doubtlessly figure out where she was, what she was doing, and come to Wes' aid.
Buffy was trying to rescue someone who she loved. Faith was trying to commit suicide by vampire.