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Some politicians are guilty of mass murder and war crimes under exactly the circumstance you give. And part of the reason those from more powerful countries are not charged is they have made every effort to put themselves above international law. For instance, Bush Jr signed the Hague Invasion Act - https://en.wikipedia.org/wiki/American_Service-Members%27_Pr... - to guard against this very risk.

edit - And in your second scenario, that would be negligence causing a homicide. It wouldn't be murder unless there was an intent to kill the hostage.