Phil Connor (and the Man)
Connor is Jurgis's personal enemy. When Jurgis hears that Connor has raped Ona and forced her into occasional prostitution, Jurgis flips out and attacks the guy physically. Even when Jurgis is sitting pretty with a relatively secure job as a boss in one of the meatpacking plants, Jurgis sacrifices all of that to smack down Connor a second time. Jurgis's confrontations with Connor cause him to lose his house the first time around and to lose his sweet job a second time around, driving Jurgis straight to socialism.
As awful as Phil Connor undoubtedly is, he is still a symptom of the larger problem. The reason that he is able to exploit Ona and use his influence to squelch Jurgis is that he has power and Ona and Jurgis have none. In a system where some people have lots of power and other people have no resources at all, of course there are going to be horrible abuses (argues Upton Sinclair). Connor is capable of doing what he does because we live in an unequal society. So the real problem isn't just one evil dude, Phil Connor; it's The Man. It's the whole economic structure that concentrates wealth and power into the hands of a few individuals while leaving the rest of us out in the cold (according to Upton Sinclair).