The Luftwaffe never developed a heavy bomber force. Partly that was because the prevailing doctrine saw the Luftwaffe as a tactical rather than a strategic force. It's intended role was to support the army, thus the favouring of dive bombers and twin-engined medium bombers. And partly it was because the champion of developing a long-range heavy bomber force, General Walther Wever, was killed in a crash before the war, and the program essentially died with him.
So, the Allies bombed 'safely' did they? I'm not sure how, for example, having 69 American bombers shot down in the March 6, 1944, raid on Berlin (9.8% of the attacking force), or 95 British bombers shot down on the March 30/31, 1944, raid on Nuremberg (12.1% of the attacking force) is considered 'safe.'
The benchmark for sustainable losses in the bombing campaign was 5%. But a constant 5% loss rate is actually enormous. If you start with a force of 100 bombers and lose a constant 5% of them on each mission, after just nine missions you are down to 63 aircraft. That's 37% of your force gone in just nine missions. Doesn't sound particularly safe to me.
RAF bomber crews were required to fly a tour of thirty missions. At an average loss rate of 5%, a crew stood a 60% chance of completing 10 missions, a 37% chance of completing 20 missions, and just a 22% chance of completing all 30 missions. Doesn't sound safe to me at all.
So, crews stood a very good chance of being shot down well before completing their tour of duty. But what were their chances of escaping alive their downed aircraft? From January-June 1943, on average the member of a Lancaster crew stood just a 10.9% chance of surviving being shot down; a Halifax crew, 29.0%; a Wellington crew, 17.5%. This especially doesn't sound safe to me.
Germany had plenty of day and night fighters to defend its airspace, not to mention radar networks and numerous flak batteries. In regards to the last, in March of 1942 there were 3,970 heavy flak guns defending German cities (at a time when the RAF had not even yet launched its first so-called 'thousand plane' raid). By September 1944 the number had grown to 10,225. Indeed, according to Albert Speer (who certainly was in a position to know these things), of the 19,713 88-mm and 128-mm dual-purpose anti-tank/anti-aircraft flak guns produced between 1942 and 1944, only 3,172 (16%) were allocated to the army for use in the anti-armour role. The rest were pointed skywards at Allied bombers. (Which was a fortunate thing for Allied armoured forces; that's over 16,000 fewer excellent anti-tank guns pointed at them.)
If by 'specialized in mass killing' you mean 'crippled Germany's ability to wage war' then yes.
Now, was the RAF's nighttime offensive as decisive in crippling that ability to wage war as was the USAAF's daytime offensive? Certainly not. The Americans went after specific industries to cause specfic economic disruption; the British went after cities to cause general economic dislocation and disruption (since hitting the area of a city was often the best one could hope to do at night, as I stated previously). The former is naturally easier to measure than the latter. But the RAF's nighttime bombing offensive most certainly had some signficant indirect effects on the German economy.
Those indirect effects, combined with the direct effect of the American bombing, devastated the German economy and destroyed its ability to wage war.
Do recall what I said many posts back (and which you have not challenged): no civilians, no economy; no economy, no military; no military, no war. That equation remains unchanged. You can continue to make the civilian/military distinction, but in an industrialized nation-state with the military utterly dependent on the mass production of that nation-state to survive and operate, such a distinction is, at best, purely arbitrary.
By the way, what precisely is a 'civilian' target?
That's pretty much why I'm doing it.