He was in the wrong place at the wrong time. He should not even have been there.

[ Which doesn't make it any better.

He's also leaving out some notable parts of that story, including that said assassin had been fond of d'Artagnan. Including that when she told him what she'd done, she had said she was doing him a favour.

Including that he thinks she actually was. These are the darker, more selfish parts of his heart that he aims to show no one. ]

You asked how their marriage ended. Death is the only way it could.