I can tell you how it happened. It happened after 15+ years of a man refusing or unable to face the truth about his own life and his ability to function within it while training himself to be more and more dangerous and while embracing his personal problems as a life manifesto.
It literally does not matter what happened leading up to it. Nothing that can be said can make his unreasonable act reasonable. Even if she had laid a trap, called him over and had her lover try to murder WM... WM would have ceased to be a victim the minute she was disarmed and he decided to then murder her. But that's not what happened. Instead we've got a violent man with a history of abuse and possessiveness and a girl with whom they've had many starts and stops. Anyone who thinks that their on-again-off-again relationship is an indicator that he thought they were getting married, and not consider the possibility that EVERY abusive relationship has an on-again-off-again quality because the abuser refuses to let go, is either being willfully ignorant or hasn't met enough people in the world.
He's had to lie to himself for his entire life. As a child that's an understandable way of coping. As an adult it's delusional. He's literally spent decades twisting the reality of life into things that fit his delusions. If she's lied to herself about anything, you can put forward the argument that it's that she can sell her body for money and not debase herself. In comparison, he firmly believes that he can abuse people (and her specifically) for years and attempt to murder her and still be the victim. You're stating that the relatively minor lies she has to feed herself and the world to enforce her delusions make her less reliable than WM, who has to twist the events of the world like a true madman in order to fit it into his belief system. By virtue of the sheer amount of lying he has to do (to himself, to everyone) to believe what he does means he is a much more dubious source than she is. His suicide note, while having lovely penmanship, is the work of a man truly out of touch with reality. He is not a reliable source on anything that happens in the real world.