Well, Ink, I think that was part of the point - and the genius - of how Lewis wrote the story. We can all see a bit of Mark in ourselves; for that matter, we can all see a bit of Jane in ourselves. Yes, he is prey for the profoundly evil people, but in allowing himself to be drawn in and preyed upon, he becomes one of the profoundly evil people - or at least gets well along the way before he's rescued.
Many of the themes in Strength can be found in one of Lewis' essays called The Inner Ring, found in his collection The Weight of Glory. In that essay, Lewis points out that nobody starts out with the intention of becoming bad. Nobody gets up in the morning and says, "I think I'll do something really evil today." Yet some of us become evil, and Lewis explains that one of the ways this happens is by our trying to fit in, to belong, to "go along to get along". This acquiescence can cause us to go places and do things and accept circumstances that, if left to ourselves, we would not tolerate.
Lewis demonstrates this poetically in the character of Mark Studdock. Notice that even though he was in many way the victim, the dupe, the sap, he nevertheless participates in the evil. He willingly and knowingly takes part in a conspiracy to engineer a riot in order to facilitate an illegal seizure of power. He's privy to the scheming and machinations of the Belbury crew to overturn the laws of England and seize, effectively, dictatorial power. Had he even an ordinary moral sense, and a shred of backbone to strengthen it, he would have stormed out of Belbury and sounded the alarm about what was going on there. Instead he acquiesces and goes along, becoming corrupted in turn (Dr. Dimble's conversation with him in Dimble's chambers at college bring this out beautifully.)
It's tempting to look at Mark with a theraputic eye - poor lad, only wanting to fit in, pushed about and maniupulated by the Fairy and Wither and all. Part of Lewis' art is that he encourages that sympathy. But don't forget the self-revelation Mark has when he's locked in the cell at Belbury: he sees that he has trashed his whole life due to his own sinfulness. He, in his foolishness and vanity and pride, walked away from good friends and what he really loved in life to pursue emptiness. Yes, he's been manipulated, but ultimately the responsibility for his predicament rests with him. He truly reaped what he sowed. That's the point of the recognition and repentance in the cell. In the end he's not "cured" by overcoming his weaknesses and insecurities; he takes responsiblity for his sins, repents, and is (barely) saved from destruction.
Strength is a wonderful parable for us all in the nature of evil and the ways of corruption. It always starts out innocuous, trivial, some banality hardly worth bothering about. But it can end up in the Objective Room, or taking orders from a demon-possessed head, unless we obey the Moral Law and act with courage. That's what Mark needed to do, and that's what we need to do.