You are thinking about the normal response to such a situation, yet you must think about this from the point of view of someone who was abused, both psychologically and physically. Those who have not faced abuse, often tell victims to ‘just speak up’ or ‘just leave’.
I was a victim of abuse myself when I was younger and many are often shocked that I suffered it for 16 years without ever telling anyone or making an attempt to leave. His abuse robbed him of the ability to deal with situations rationally. We cannot judge a victim of abuse for their inability to act in a way that we think is ‘logical’ or expected.
Many victims feel trapped, even if they are not physically imprisoned and sometimes the emotional torment can build up to the point that they mentally break and do something drastic. I am in no way condoning his act of murder, yet I can understand why he did not simply leave as you suggested he should have.