trigger_hurt doesn't have a flag that will kill all clients so you will have to build trigger brushes in those areas where you want the players to be killed (set them to "Start disabled" with damage 1000). Then place one or more trigger_multiple that will activate the trigger_hurt when you step into it. Add outputs that will enable and disable the trigger_hurt. If you don't know what I'm talking about find some tutorials about the input/output system - sorry that I can't provide you a link right now.
I don't know if there's another possible way to do this.
I've never tested this with ubercharge. How is it in cp_well, can you be killed by the train when ubered? (They don't use trigger_hurt but I guess if that works it will with trigger_hurt, too)
Use a point_hurt, it's a point based entity that lets you set a radius to hurt people within rather than tying it to a brush volume. Making brushes all over your map is cumbersome and arguably inefficient.
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