Marxists do not judge historical political characters accordingly to whether they have been good or bad people either in some ambiguous idealistic moral political sense or in the individual sense of being good or bad to his family, friends and all. What matters is which class the politics of the person in question represents.
Now, with Stalin we are talking about the chief representative, the face of a regime that murdered thousands of communists and revolutionary, militant and class-conscious workers, the overwhelming majority of the Bolshevik Party of 1917 as well as the overwhelming majority of the central committee of the Bolshevik Party of 1917. He was the face of a regime that in every ideological move it made, in every policy it announced did everything within it's power to attack, weaken and cripple the revolutionary consciousness obtained by the working class who made the October Revolution. The face of a regime that viciously attacked the internationalism of the world workers' movement by putting forward the theory of "socialism in one country", advocating Russian nationalism and turning the communist parties in the world into mere instruments of it's interests by destroying even the tiniest dissidence in those parties. The face of a regime which were defenders of those who managed factories, those who commanded armies, those who were high in the hierarchy of bureaucratic institutions. The face of a regime that was imperialist, mobilized millions and sent them to death for it's imperialist interests and occupied half of Europe and formed satellite states there.
He was the face of the reactionary, counter-revolutionary regime in Russia, he was the face of the Russian bourgeoisie rising from it's ashes. He was the gravedigger of the Russian Revolution. Whether he was good person or a bad one, whether he had good intentions or was malicious is hardly relevant in the face of the class, the regime he represented.