We are talking about the 1940's you are aware, right?
At that time "precision bombing" pretty much meant that your bombs landed in the same county as your target. Even the best bombsight of the era (the Norden) was accurate only to an ideal of "2% of altitude".
That means that a perfect aimed and operated B-29 would have it's bombs drop within 2 kilometers of it's target simply because of issues with accuracy.
And this was the most accurate bombsight of the era!
There was no other way to attack such industrial facilities.
I think parts of this discussion are based on wrong assumptions. British Air Marshall Arthur himself said the following about the bombing strategy:
The destruction of German cities, the killing of German workers, and the disruption of civilized community life throughout Germany [is the goal]. ... It should be emphasized that the destruction of houses, public utilities, transport and lives; the creation of a refugee problem on an unprecedented scale; and the breakdown of morale both at home and at the battle fronts by fear of extended and intensified bombing are accepted and intended aims of our bombing policy. They are not by-products of attempts to hit factories.
On the American side you have people like USAAF General Anderson, referring to the bombing of militarily unimportant towns in 1945:
A note by US air force general Frederick Anderson to his press office notes that such operations were "not expected in itself to shorten the war ... However, it is expected that the fact that Germany was struck all over will be passed on, from father to son, thence to grandson; that a deterrent for the initiation of future wars will definitely result."
Allied bombers chose 'easy' German targets | UK news | The Guardian
It is important to note that especially after D-Day, many cities in Germany were not bombed because there was anything militarily valuable in there but simply because they were expected to (and did) burn well, especially when there were medieval town centers. In the case of Darmstadt, as in many other cities, the main industrial areas were not even hit. In this kind of bombing, the bombers would fly over a landmark in the city and spread out from there in a fan-like formation to spread first explosive bombs and then incendiary bombs across an area of the city which had been designated with fire risk maps and population density maps in order to maximize the damage to the civilian population. Darmstadt was a testing ground for that method of flattening cities and it was later applied to other cities in Germany and Japan.
Regarding the argument that the bombing accuracy was limited and therefore the carnage that was the bombing of Darmstadt was inevitable, it may apply to some bombings earlier in the war (when the German fighters still shot down every second allied bomber and therefore the bombers had to fly at night and in high altitude). However, in September 1944, the allies already had air superiority, they could basically do whatever they wanted. Precision bombings WERE possible at that stage of the war. And they were very successful. For example, bombing oil refineries led to German tanks on the Eastern front not being able to move anymore because they didn't have any fuel. Using their bombers to support the advancement of troops was also possible. Area attacks on cities, on the other hand, were not done despite the damage they would do to civilians, but because of it, at least at that stage of the war.
If you want to see a high level discussion between historians about the bombing of cities in WW2, here is one:
The Allied bombing of German cities in World War II was unjustifiable - YouTube
Note that all four panellists agree that the flattening of German cities into 1945 (raids for which Darmstadt had been the testing ground) was not
A nice BBC documentary about the WW2 bombing of civilians, with Würzburg as an example for the militarily senseless bombing of residential areas, and an explanation of how targets were chosen.
Timewatch - Bombing Germany (BBC 2001) - YouTube
I don't think anyone here (possibly except for Graffias) would compare the atrocities of the allied air forces to those of the SS. And if the damage done to German civilians was inevitable for hitting military and industrial targets with the technology that was available, I would be the first one to justify those bombings in the context of WW2. But deliberately bombing the civilians for the reasons stated by Anderson, which is a fancy way of saying lets kill as many of those bastards as possible was indeed pre-meditated mass murder. And as such it should be judged. Where in the allied bombing campaign the line between legitimate strategy and simple mass murder was crossed is hard to decide, I would propose that it is crossed when civilians are explicitly designated as targets and their killing is developped and optimized in a scientific way. If you feel uncomfortable judging people from the allied side and prefer to assume that they would have acted both as morally and as effectively as possible, that's okay, but one point I find important: Today, i.e. at a time when nato countries like to "take responsibility on security and stability issues", the deliberate bombing of civilians should clearly be on the list of things which should not have happened and should never happen again. As a German, I do not want to be part of an alliance which considers punishing civilian populations for their evil regimes to be a justifiable thing to do.