The Important war crime/collateral damage distinction: International Law recognises that not all forms of civilian casualties constitute war crimes. Hence the distinction between a war crime and collateral damage. To delve into this deeper the reason being is because international law recognises the balance between humanitarianism and military necessity. There are times where humanitarianism is the number one principle that has to be maintained. But other times where military necessity outweighs the humanitarian principle. Luis Moreno Ocampo, the former Chief Prosecutor of the ICC who helped prosecute the U.S backed military junta in Argentina stated Under international humanitarian law and the Rome Statute*, the death of civilians during an armed conflict, no matter how grave and regrettable, does not in itself constitute a war crime. International humanitarian law and the Rome Statute permit belligerents to carry out proportionate attacks against military objectives even when it is known that some civilian deaths or injuries will occur. A crime occurs if there is an intentional attack directed against civilians (principle of distinction) or an attack is launched on a military objective in the knowledge that the incidental civilian injuries would be clearly excessive in relation to the anticipated military advantage (principle of proportionality)*
Chain of command when it comes to military decisions: When it comes the U.S government, or any other government as well as the military there is a chain of command. This means that even if a violation of the law takes place, it doesn't mean that was authorised. To give an example, lets say during D-Day Eisenhower authorised a siege of a particular town, and during that siege individual allied soldiers went and committed massacres, does that make Eisenhower himself the war criminal? No. It makes those soldiers the war criminals. When applying this to Obama people need to recognise that not every single strike that was authorised during the Obama was authorised by Obama himself. I read something in passing(it could be mistaken) that as many as 70% of the strikes weren't authorised by Obama himself and that they were authorised either by the military or the CIA. So even if a case could be made that many of those strikes were war crimes, it doesn't mean Obama himself was a war criminal because you'd have to prove Obama personally authorised strikes that constitute war crimes.
Mens rea as a princple: This is an extension of point two. Mens Rea is a legal principle that states that person's mental state is important when considering if something is a crime. The reason I think this is important is for this reason. Firstly, is there any evidence that Obama intentionally sought to kill civilians with drone attacks? Secondly, if it could be proven that Obama authorised a strike that resulted in civilian casualties, could you actually prove that Obama new there would be civilians or that he was briefed on on there being civilians and did it anyways. The first to me is nearly impossible to prove and the second is incredibly difficult. You have problems ranging from terrorists using human shields(which did happen during the drone war), to local allies and actors such as Pakistani intelligence in the Waziristan region giving sometimes false or misleading tips to even the military and CIA officials giving information in briefings that's not always accurate.
My conclusion is this. I do think during the Drone era there were war crimes that were committed because of tactics such as Double Taps. And I still remain a major critic of Drone Warfare However I think its very difficult to prove Obama himself would be a "war criminal" because of all the complicated mitigating factors both militarily and legally under international law. So I come down the "vast oversimplification side".
from a reddit debate, much better points than you hacked up from grayzone