Why was Stalin worried after he learned the USA had used A-Bombs against a pair of enemy cities? Didn't 'Bomber Harris' kill more people?

The art of fire-storm bombing was perfected on Japanese cities, and the big raids killed more people than the atom bombs did. The same technique was then used on Korea, which was more or less bombed back to the stone age.

Because It took Bomber Harris along with the USAAF (United States Army Air Force) about 4 years of almost constant round the clock saturation bombing of German cities with thousands of aircraft to do what took two just US aircraft with one bomb apiece dropped just days apart, that's why he was worried, and with good reason.

Stalin wanted to use the opportunity to take over as much ground as he could.

For the record, the USSR was NOT at war with Japan until AFTER the dropping of the bomb; Stalin was happy to let everyone else do the work and try to take over land with minimal effort. The bombing meant that Japan may surrender quicker than he expected and he rushed to declare war and invade Manchuria.

Stalin did not care about the victims, he just wanted to have an as large as possible territorial gain as he could.

Thats why he got as many German scientists as he could.

The raids on German cities involved up to a thousand aircraft dropping tens of thousands of bombs to accomplish the same damage which could be done by one aircraft armed with one bomb.