[adventured]

That's pretty simple. bin Laden's organization was a direct threat to the US. Pakistan was protecting him. The primary responsibility of the US President is not to respect the sovereignty of Pakistan, it is to ensure the security of the United States and its people. For the US President, respecting Pakistan's sovereignty sits below ensuring the security of the people of the US in their job description.

Besides that, international legality is defined solely by the countries with the biggest sticks. That will always be true, without exception. The UN and all global bodies of justice and rule setting derive their authority and possess capability solely at the permission of the most powerful nations.

The world isn't run by the UN, international legality isn't defined by 195 nations, it's all run by six or seven countries (which control the majority of all economic activity and by far have the most powerful militaries). The UN is a lever for those powerful nations to keep order in dealing with all the others. It's a means to herd cats.

There's a quote by a rather vile heiress named Leona Helmsley, paraphrasing: taxes are for the little people. Respecting sovereignty is only a fixed rule for the little countries. Why? Because of how consequences work and who can dish them out. I'm not advocating that, mind you, I'm saying that's actually how the world works and always will. You see it with every empire that has ever existed, you see it with the US, Russia, China, etc.