I've learned from the VA that this common misconception has more to do with some vets lacking healthy coping skills after all they've experienced. They've been to hell and back and have been taught to stuff away all they've seen and done, as a good solider must carry on with the fight ahead of him and not look back. While this mentality might serve them in the battlefield, in a civilian context, all the sh*t that's been buried eventually makes its way to the surface and manifests in some way or another.
Some vets, not knowing where to turn for help or simply not wanting to get it, instead resort to more destructive ways of helping numb the pain-whether drugs, alcohol, or meaningless sex- even if it's just to forget it all for even a moment. Sometimes the family unit can already be compromised after returning home from a deployment, and with so much discord both within the family and within the sufferer, the unbearable amount of strife for all involved can set up sort of the "perfect storm" of circumstances for things to head south.
My combat vet cheated just once on his then-wife who he was already separated from, and he swore that no matter how f*cked up he ever happens to get in the head again, he could never do that to anyone ever again. It's been over 10 years, and he hasn't done it to anyone again since. So yes, sometimes good people who are in really bad places can make mistakes. What counts is what they learn from them.
And the VA is also doing a lot of hard work attempting to steer vets towards healthy and lasting forms of healing that are beneficial for both them and their families. I hope and pray that they continue to do the hard work of helping vets find emotional freedom, and that vets know and understand that they are NOT alone and that there is SO much help readily available that will help save them and their families.