Neither Trotsky nor Stalin could have substituted their own will for a democratically planned economy - the difference is that Trotsky knew that, and Stalin didn't.
I am not sure that Stalin set out to starve people, either. The issue was not whether one man was in charge or not, but under what conditions the Soviet Union industrialised. Bukharin (with Stalin's agreement at first) wanted to proceed at a 'snail's pace', through trade with the countryside. It was Trotsky who said that this was a recipe for disaster, and that the peasants would hold the cities to ransome. There is little doubt that Trotsky's policy (Preobrazhensky's in the first place) would have been hard on the peasantry, resting on forced transfer of resources to boost production.
The difference was that Stalin, rather than take control of the situation let it slide, so that over time the trade policy created a goods famine in the countryside (as the cities were not productive enough to re-tool them) and starvation in the towns (because the peasants were not trading grain for goods). Faced with the 'scissors crisis' Stalin reversed the Bukharin policy and imposed collectivisation on the masses. Some Trotskyists (like Preobrazhensky) were confused, thinking that Stalin had come over to Trotsky's way of thinking, but Trotsky himself (who wrote extensively on the tragedy of forced collectivisation, but was understandably also concerned with the annihilation of his political supporters) accused Stalin of reacting to events too late, and so too harshly.