I agree that there is no issue with killing enemy combatants on the battlefield. Combatants who surrender are normally given prisoner-of-war status. In the case of allegedly illegal combatants, they can face a criminal trial or military tribunal.
France seems to be working to avoid that possibility by killing as many of its citizens before they surrender. The French may insist that any who manage to surrender to the Kurds or other forces face trial in Syria or Iraq rather than returning for a trial in France.
An issue for the US is that the Obama administration developed a very broad interpretation of battlefield and combatants. Essentially it claimed the right to kill anyone at anytime if an internal review within the administration concluded that they were a terrorist threat.
The US has certainly violated the laws of other countries in the war on terror. For example, the Bush administration captured an Egyptian cleric in Italy and returned him to the US in violation of Italian law. The CIA agents involved were convicted of kidnapping in Italy; one was recently pardoned:
The apparent Russian poison attack against their former agent takes things a step further. The US prefers drone attacks or kidnapping rather than poisoning.
Is the outrage in the Russian attack because of the use of poison instead of explosives?
Would everything be okay if the Russians used kidnapping or a drone attack instead?