If you send the Seals into Sweden, to "take out" anyone withouth the consent of the Swedish government, you tend to provoce what is elsewhere known as a act of war.. But of course Sweden wil never go to war with US, rather it wil make a hell at UN and other international body's and it wil be a time when the diplomatic contacts between Sweden and US wil be on a all time low.
The best idea wold be to let the Swedish government, do what is in their interest, and let the law of the land be respected. Even if you do not like it.. To send in the Seals or other military groups wil be counter productive to what you really want. If US was to kill Assage it wil not stop the leake, and it wil not stop groups like wikileak and other organizations to prove their point. It wil rather just prove for everyone with half a brain, that Assage was right, and the work of Wikileak et all was worth it.
From a government point of wiew, the world was far more easy before the invention of internet.. From everyone elses, the world are far more open now, than it was before the invention of internet (Im old enough to rembember that news was old news when it was news, today you can get news, when it is fresh news)