Even if there had been a properly declared war, international law provides that one may not assassinate the leader of one's enemy. - Richard. Bridge

There is no such international law. You're speaking off the top of your head with obviously no knowledge of what you're talking about.

"Osama Bin Laden ... was an appropriate military target. As the titular and spiritual head of al-Qaida, he was the functional equivalent of a head of state or commander in chief of a terrorist army. From the beginning of recorded history, killing the king was the legitimate object of military action. The very phrase 'check mate' means 'the king is dead,' signifying the successful end of the battle." - Alan Dershowitz, Constitutional law scholar, renowned criminal and civil liberties attorney

http://www.hudson-ny.org/2093/targeted-killing-vindicated