President Drone, war criminal

which, otherwise, disagree with one another, as sourcing for his point that said usage is murder... My point, which was likely quite obvious, is that the author of this article specifically chose to include civilian death rates from the study that placed the highest percentage on said deaths, while failing to mention the death rates from the other studies, for the purpose of making his point that the use of drones equates to "murder." [This quote of yours is actually taken from two different posts, but I don't think posting them together here distorts your meaning]

By "sourcing for his point" I take it you mean something like this:

substantiating the ethic that "drone use = murder" by grounding it in evidence taken from the three sources. 

First off, I don't think McNamara intended that these sources provide the substantiation for his ethic that drone usage equates with murder. I think he established that ethic [to his satisfaction at least] via the Orwell essay coupled with his recounting of the death of the American teenager via drone attack in Yemen. The main thing he intended with the three sources was to establish that murder by drone is, in fact, going on, not that it is murder. And, as long as we are guessing as to why he quoted only the highest death ratio of the Brookings op-ed [and please note that I have agreed he should not have used the quote in the way that he did], my guess would be that a) he was trying to get the attention of an American readership numb to the death of a single Pakistani, but a readership that might perhaps pay attention to larger body counts, and b) he wanted to cite an authoritative American source that not only gave a high body count but also argued that a high body count is acceptable. I would guess that McNamara did so to highlight the barbarity of a mind that would accept a 10/1 ratio. In fact, whether or not the 10/1 ratio is actually the correct ratio is irrelevant in this context: it is the acceptance of it that is barbaric. Anyway, that's my guess (Remember for McNamara the taking of even a single life is unacceptable.) [By the way, in 2009, just prior to the publishing of the Brooking's op-ed, there was at least one well-documented report which presented much higher ratios] and,

The point is that all three sources--in different ways--document what matters to McNamara: murder is happening. So I don't have a problem with his sources not being in absolute agreement with each other as to the total numbers of civilian deaths, or for that matter that one study says that the civilian/insurgent kill ratio is improving [though McNamara should have mentioned it], while the other study does not. 

In other words, from McNamara's point of view, similar to those who oppose capital punishment, even if drones killed only one person over the course of a year or more it would still be murder.