Somewhere in my readings on MLC I came across an article on the net called something like Seratonin, from bliss to despair, by a Dr Carver. It explained how depression effects certain changes in the brain, which lead to ... well, basically, all the symptoms we notice in our WAS. Essentially, when seratonin levels become severely low is the time he characterizes most affairs of happening--and that the brain misfires the worst.
I'm not saying that this excuses the depressed person from having an affair, especially because not all moderately or severely depressed people make that choice. However, I think we have to have compassion for the fact that their brains are not functioning the way they did before some traumatic event (be it the sudden death of a parent or friend, or some trigger to earlier abuse or childhood trauma) occurred, and that they genuinely have not yet developed the skills to take ownership of the chaos in their heads (and to which they contribute).