It would be so easy to tell the police, to tell John, that her parents were murdered by Dean Van Halen; John suggests it, in fact, because the only reasonable explanation he can see is that Mary killed her father after he killed her mother and tried to kill John, and he doesn't want Mary to take the fall (the final plan, to conceal her father's death until a few days after Deanna's death by a supposed slip on the stairs and to tell everyone that Samuel died of a heart attack brought on by a broken heart, is John's; Mary has trouble shaking the thought that John is a hunter born). Mary knows even before John mentions it that Dean makes a marvelous scapegoat: he was at the scene of both deaths, after all; enough people saw him to be sure he was both in town and new in town; Mary doesn't expect anyone in Lawrence or environs will ever see Dean again; if Dean hadn't been there, Mary and Samuel would never have known to visit Liddy Walsh and Samuel would never have been possessed, and the demon would have left before anyone could confirm there was a demon in the area, so he killed Samuel and Deanna as surely as if they died of bullets from his (misaimed) gun.
Hunters have an unspoken code of honor—be truthful with one another, because you never know when you'll wager your life on another hunter's story of what killed a similar beastie and you never know when your own experience will be the difference for another hunter between life and death, and try not to blame one another for unintended consequences, because with unintended consequences by definition you never know—but Mary's no longer a hunter and she's hurting too badly to care about her honor, and accusing Dean Van Halen is the obvious, the safe, the only thing to do; she can't bring herself to do it, and she has no idea why.