If they kill Lisa, you've still got Ben hanging around. They have to kill him too, which, odious kid or not, I don't want them to do, if only because they'd act all dramatic about Dean's devastation for about two seconds...followed by four weeks of har-har slapstick. If they don't kill Ben, then he's on the road with Dean and Dean essentially becomes John which, no thanks.

I can see them going the noble-sacrifice route, even though that would require us to handwave the fact that Dean's departure from Casa de Lisa would in no way protect her. But we've all gotten very good at handwaving over the years, so much that I think they now expect us to do it.