The state wouldn't be killing anyone. This would be the domain of an invidual and his family. The question is does the state have the right to stop them.
And, throughout American history, of course the state has the right to prevent the killing of its citizens.
A rather large 'duh' factor there...
Again, by sanctioning and codifying assisted suicide, the state of Oregon is complicit in the killing of the innocent under its legislation.
Without the cooperation of the state, assisted suicide is punishable by law. Under Oregon's law, the state is assisting the assist.