What did she think was going to happen?

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Just saw this via streaming.

There are threads that assert that Mariana/Sylvia was just trying to scare the miscreants in the trailer. Given her cool and analytic behavior, having manipulated the propane fittings then starting the fuse it seems unlikely she was expecting "candles on a birthday cake" but knew full well what would happen.

That said, sitting by his bedside in the hospital she did seem contrite when confessing to her daughter's father it it was she who murdered her mother and his father.