Good and evil can exist side my side in people and frequently does. I see no dichotomy between the two sides of JT.
JT had no power over Scott when the affair started. Scott was not forced or coerced into the relationship. The problem was, if the drama is to be believed, is that Scott fell deeply in love with Thorpe, who wasn't remotely in love with him and as far as JT was concerned the relationship ended when Scott left him to go elsewhere.
This is a pattern of human relationships that is only too common across all the combination of gender relationships, someone has a brief affair with someone, it breaks up and one moves on and the other doesn't, but remains bewitched and in thrall to their previous lover.
Give JT his due, though, despite being ferociously politically ambitious, he stuck with a party whose principles he believed in, even though their chances of getting near any power were, almost negative when he joined. Had he compromised his principles and joined the Conservative party he would undoubtedly have made very rapid progress up the greasy pole.
Homosexuality was illegal at the time of his affair with Scott, but after one election, there was a real possibility that he could have become a coalition partner. I can see how, a man in Thorpe's position at that time, faced with the prospect of power on one side and a gay lover who kept popping up like a game of Whac-a-Mole, and threatened his career at a critical moment and whose revelations could see him exiled from his whole life, his reputation ruined. At such a critical time a man in extrenis might well consider murder.