Michael is very insecure, and this insecurity leads him to do some pretty horrible things.
The guy has a carefully crafted image. Although Michael's just your average wealthy Northeasterner, he's recast himself as a European intellectual despite the fact that "he'd only spent a year in France" (12.23). At first, it's pretty funny to play the romantic Frenchman with all of the subtlety of Pepé Le Pew, but this doesn't last long.
As Michael's relationship with Helen develops, she alludes to his forcefulness and need to be "dominant" (12.58) during sex. Suddenly, things aren't so amusing. But it's not until Helen break things off with Michael that we see how insecure he truly is.
First he becomes childish, sobbing and throwing a temper tantrum because he doesn't get what he wants. Things only get worse from there, though, and he rapes Helen after she begs him to leave. The same childishness that initially seemed so amusing suddenly becomes deeply disturbing. In a cruel irony, Michael exerts his power over Helen only out of his own feelings of powerlessness. Ugh.