I don't think that people have no sympathy for LPZ, what happened to him, and how events forced him to learn Bixie, because up to then he didn't really have a choice. What he did with his skill once he learned Bixie was entirely up to him, though. He chose to blame Yue Lingshan for her father's evil, assuming that she only loved him as part of a greater plot by Yue Buqun to gain his confidence, when she really loved him even after he was blind. At the point when he actually killed her, I think he finally realised that he had wronged her, but his pride refused to let him accept that and so it would have been easier for him if she just didn't exist.
If I remember correctly he was also partly to blame for his blindness as he was enjoying his revenge too much, and if he had just killed off Yu Canghai and Mu Gaofeng as efficiently as he could he might have got away clean, but he chose to toy with them and torture them sadistically. He then rejects his wife who by now no longer had any reason to stick by him other than loving him (and it wasn't due to the castration which dulled his feelings for her - he had resented her even during their wedding - the moment he realised it was Yue Buqun who attacked him he chose to blame Yue Lingshan as well). He kills her, and throws in his lot with Zuo Lengchan, supposedly to take revenge against Yue Buqun, but ends up murdering scores of students from four of the Five Peaks Sects. It might have been fate which drove him to the point where he had to learn Bixie, but he chose his own destiny after that (even if you can say that fate had destroyed his innocence and made him paranoid), and at the point when LHC defeated him in that cave, LPZ was a deranged, merciless and uncontrollable force. If left to his own devices, he would undoubtedly be responsible for the death of many many people in the future, most of whom would be innocents. And it wasn't as if LHC hunted him down for the death of Yue Lingshan - LPZ would have murdered him in cold blood without so much as a second thought despite LHC never ever hurting him in his life.