Boyle's actions are not a result of a bad childhood or propensity toward criminal activities, they are most likely influenced by 5 years of psychological torture. I think it has to be taken into consideration that his anti-social behaviour occurred directly after release from captivity.
What you are ignoring is that Boyle's behaviour toward Coleman before they were kidnapped showed every sign of the domineering, coercive person he was. She didn't want to go to Afghanistan, yet he forced her. This has nothing to do with PTSD, but what IMO is a major personality disorder.
And the infanticide reference is irrelevant. Post-partum psychosis is real.