If you've seen the film, a speculative question...
I just saw "The Suspect" for the first time at the Noir City film festival.
It struck me that an element of Philip Marshall's decision at the end of the film to give himself up to protect his neighbor from a false murder charge was the realization that his young wife belonged with a younger man, such as his son. That's part of what I read into his decision to turn himself in. I felt that he knew, on some level, that his happiness with his new wife was destined to be short-lived. It made the ending all the more poignant.
Since the question is spoilery, don't read on if you haven't seen the film.