"And you might blame some impossible-to-calculate millions of the 25 millions of the Great Patriotic War on Stalin's criminal negligence with regard to Hitler's intentions after the Pact. (That one, Doug, you can probably pin on the Father of Peoples personally.)"
Well, no, I would blame that on the Nazi regime and the Wehrmacht personally. Stalin and the rest of the party leadership can be blamed for not being prepared (see Berezhkov, At Stalin's Side, p. 202). But it was German militarism that waged war on the Soviet people, and the blame is theirs.
In general, I would say that there is a difficulty in separating out the subjective failures of Stalinism from the objective conditions. The whole trajectory of Stalinism was that it filled the vacuum left where capitalism failed, but the working class were not strong enough to take control. Consequently, the Stalinists tended to be left in charge of countries that were collapsing, or on the verge of collapse - conditions that militated against justice and humanitarianism. So I don't think it is possible to adapt a profit and loss account of deaths or lives saved to estimate Stalinism - except of course for those directly executed in political purges. Without the control of a successful workers' state in Russia we cannot estimate how many lives lost are due to Stalinism, and with it we would not need to make the calculation.
On the other hand, the subjective failings of the Stalinists are pretty grotesque. At their root they stem from the desire to dress up failure as success. There would have been nothing wrong with conceding that post-revolutionary society was in a dire state, and in no position to make social reforms. But to dress up crisis management as the New Jerusalem, that was criminal.
Likewise, there was nothing wrong with making concessions to the market in the NEP - social reform cannot jump ahead of its material basis. The error was to adapt to the initial success of NEP and make it into a principle, rather than a strategy - until the peasants' trading activities threatened the state. Similarly, there was nothing wrong with trying to take control of the countryside. But Stalin's repression was a precisely a failure to take control. Military rule only led to a collapse in agricultural production - hence the famine.
And so too, there was nothing wrong with trying to buy time by compromising with Hitler. What was unforgiveable was the lauding of this policy as a positive contribution to human progress, rather than accepting it for what it was, an attempt to gain a breathing space. Worse still was Stalin's taking the opportunity to carve up Poland, thereby making Socialism synonymous with the division of the spoils.