I had a similar thought process about Anderton killing Sean's killer. If he's capable of resisting, then THAT should've been the vision - him nearly but not quite killing this guy. But I didn't think Agatha could "rig" a vision - not for all 3 of them.

So I was contemplating it and starting to have "ooh, but it DOESN'T work" whining but then I realized that what the trio seems to see is INTENTION and Leo (the guy pretending to be Sean's killer) had tremendous intention that Anderton would kill him. So they saw the future as it would happen, but not under the circumstances we are lead to believe will frame it.

But I agree, Agatha started the tumble of events by showing Anderton the particular vision of the drowning