I thought I had that covered by the "it really really wants to". There is nothing magical about Congress. Under this theory, that killing people is OK as long as it is authorized in advance by Congress, there is no logical limit.
You really are tilting at a windmill here. Bin Laden directed an attack on this country that killed thousands. He admitted such publicly. He was not even a uniformed soldier to where those who captured him were bound by the Geneva Conventions. He could have legally been killed by the soldiers on the spot with or without Congressional or Presidential authorization.
And yes, Congress does matter as a means to check or authorize executive power. And there is a logical limit. Don't engage in lethal terrorism against this country and you won't get greased.
It's pretty pathetic that you are unwilling or unable to grasp the concept without trying to make more out of it than what is there. There are plenty of bona fide usurpations of power going on by the Obama Admin. Killing bin Laden is not one of them.
Stuff happens. Get over it. Don't start a war.
Death to Bin Laden and his running dog lackeys!