A question

Great film, but I didn't quite understand one thing, although it could have been due to sloppy subtitles on the DVD. Near the end, Zhongliang tells his sister that he's killed her husband, Zhengda. We already know that, years earlier, he had poisoned him and reduced him to a virtual vegetative state by putting arsenic in his opium pipe. But the subtitle translation was "I just killed your husband", so I took that to mean that he had now sneaked in and actually killed him, presumably to release his sister from her responsibilities, so that she would be able to leave the family estate and start a new life.

The other possibility is that Zhonliang is actually confessing to her that he poisoned Zhengda years earlier, and thus virtually killed him. In that case, the subtitle translation is obviously misleading. But I thought his sister was already aware of his guilt, or at least strongly suspected that Zhonliang had something to do with it. At one point in the film, she says that she had "smuggled" him out of the estate after what had happened, as if she knew that he would be in danger if he remained there.

Also, I was wondering exactly why his sister reacts to this by slapping him. I guess there would have been mixed emotions but, if she already knew or suspected that he'd done it, why would she slap him like that for telling her?