A few historical precepts:
Golden Rule - "He who has the gold, makes the rules." - Who would declare this an illegal killing? Likely not any Western democracies. And, likely not any countries in which AlQ has ever operated. And, likely not any countries who have been victims of terrorist attacks. And, likely not any country with substantial economic ties to the US. That leaves NK, Venezuela, Cuba and a handful of others. They might get up and yell at the UN. But, that's as far as it'll go.
Recorded history - "The winners write the history books." - Those people likely to call this "murder" are not going to be the ones recording the accounts of this episode. The US (and allies mentioned above) will be the ones to craft the version of history that is recorded, and it will say OBL orchestrated the attacks on the US, and then he was killed by the US. That's about it.
But, to answer your question, it wasn't a political assassination. Although I don't agree with the war powers granted the President, he was authorized to kill OBL any time he wanted.