Stalin was an amazingly bloodthirsty and vindictive man, but I think it's not beyond the bounds of possibility that elements within Soviet society or the army were, at various times, plotting against him. He had a lot of enemies. So, were any of his persecutions 'justified', strategically if not morally?
It's hard to say. It's certain that until the Purges, Stalin was not an absolute dictator with a secure power position. Despite being the clear dominating figure within the Party by the late 1920's, he was still at the mercy of the Central Committee and to a lesser extent the Party Congresses. After Lenin's death Stalin's future as GenSec was very much in jeopardy with frontrunners like Trotsky, Bukharin, and Zinoviev vying for power. Stalin's political maneuvering during the 20's and early 30's (first allying with Zinoviev/Kamenev to take down Trotsky, then allying with Bukharin/Rykov to take down Zinoviev/Kamenev, then taking down Bukharin/Rykov with none left to oppose him) was very much a necessity for him to maintain his position in all likelihood.
However after the destruction of the "Right Opposition" (Bukharin/Rykov's faction), there was little real political threat to Stalin and there's no evidence the Red Army was remotely disloyal to the Party (the earlier Red Terror and Civil War had rooted out military opposition to Bolshevik rule). Sergei Kirov, despite being utterly loyal to Stalin, was nonetheless seen by many within the CPSU as a Stalin alternative. This climaxed in the notorious 17th Party Congress in 1934 where Stalin received far more negative votes than Kirov (despite winning handily and retaining his position). This was a manifestation of dissatisfaction within the CPSU of Stalin.
Could this anti-Stalinist bloc have used Kirov as a figurehead and taken Stalin down? Very unlikely, in no small part because Kirov wanted no part of it. Nonetheless Kirov met with a [rather shady murder] (http://en.wikipedia.org/wiki/Sergei_Kirov#Assassination_and_aftermath) and that was used by Stalin to kick off the purge of the CPSU. By 1938 there was no question Stalin was secure as an absolute dictator.
A perfect book to discuss Stalin's powerbase, opposition, and mindset running up to the purges is Montefiore's [Court of the Red Tsar] (http://www.amazon.com/Stalin-The-Court-Red-Tsar/dp/1400076781)
Well, you aren't paranoid if they are plotting against you. I think that the "destalinisation" that followed his death shows that he wasn't exactly surrounded by faithful allies.
It's a short answer as I don't have English sources, but that's my understanding.