Lars Thorwald is a traveling salesman in an unhappy marriage who kills his wife in order to run off with another woman. He would have gotten away with it, too, if it weren't for those nosy kids—a wheelchair-bound photographer and his buddies.
For a guy who's at the center of the film's plot, Thorwald doesn't get a lot of screen time. Until the very end, everything comes from Jeff's observations. We see his activities from a distance: leaving the house with weird packages and scrubbing down his apartment after his wife "leaves," along with other suspicious behavior that get Jeff all up in his business. The "other woman" angle comes from secondhand information courtesy of Doyle and a bit of assumption from Jeff and Lisa.
That's it for almost the entire film. We never hear his voice, we never see his activities, and we can't even be sure he killed his wife. We see only what Jeff sees of him, a smart cinematic technique that draws us in to the drama. As a character, Thorwald has to remain very much a man of mystery because of that.
Only after Thorwald realizes he's being watched does he make an up-close appearance. (And seriously, how freaky a moment is that? Watch it with an audience, and they're apt to shriek like banshees when it happens.) He soon shows up in Jeff's apartment, and there at last we start to see what kind of person he really is. He seems more upset about being watched than about carving up his wife.
In fact, he's a little pathetic. As he talks to Jeff in the darkened apartment, admittedly right before trying to kill him, he comes across as a trapped animal:
THORWALD: What is it you want? A lot of money? I don't have any money.
The scene suggests that he's something more than just a horrible monster (though to be clear, he is that, too). He commits murder because he wants to be free of his miserable life, and now that he's been caught, he's not ready to face the consequences. This isn't murder on a grand scale; he's just a poor schlub trapped in an unhappy life that he thinks he can get out of simply by getting rid of the wife.Is a little happiness too much to ask?
There's something sad and a little depressing to that. It's also staggeringly ordinary, a miserably normal life taking a bloody turn. Hitchcock always had a little sympathy for his villains.