2. Kanaris (nazi intelligence director) said after 37 their spy network didn't exist any more in Russia.
While you can't deny that there were innocent cases, one can't deny that many people were sued for a reason, as Stalin knew the war was inevitable and was doing anything to support the state.
Another interesting story is that as a spy you could get a smaller term than for some other crimes, so criminals happily became spies and local authorities happily reported their success to Moscow.
As for "dark rotten times", in the 20's Stalin was talking to English ambassador who asked Stalin if there would be a world war and Stalin replied that there would be a war and it would be much worse as with Germany, Japan would also attack from the east.
All actions of 20-30s and Stalin's decisions should be estimated from the war preparations that started in the early 20s. Young Soviet country didn't have friends and did everything it could to survive. Were Stalin's methods brutal and inhuman? Probably. Would civilization as we know it exist now if Hitler managed to get rich Russian resources? No. So sometimes ends justify means.
Just my point of view based on numerous historical books I read.