Read Article 2(4) of the UN Charter. The violation of international law results from the failure to "respect the sovereignty, territorial integrity and political independence of other States." So again think of my hypothetical above, with the shoe on the other foot. If another country conducts a military strike in the US, and violates US sovereignty, the claim of self-defense must be against the US (i.e. the member State), not a claim of self-defense against an individual residing in the US.
Essentially, there would be no rule of law if Use of Force was authorized, under the self-defense, so long as one Country alleges a terrorist is residing in another Country. To be fair, under these specific set of facts, it may have been possible to make a case before the Security Council and have the Security Council authorize the Use of Force. However, that is the whole point the US circumvented the established law and acted unilaterally, when they could have gone to the UN presented their case and had a vote on the merits.
Essentially, there would be no rule of law if Use of Force was authorized, under the self-defense, so long as one Country alleges a terrorist is residing in another Country. To be fair, under these specific set of facts, it may have been possible to make a case before the Security Council and have the Security Council authorize the Use of Force. However, that is the whole point the US circumvented the established law and acted unilaterally, when they could have gone to the UN presented their case and had a vote on the merits.