Not true. There are plenty of rational evil people. They only need to take one simple sentence to heart to unlock evil.
“The ends justify the means”
Infowarrior has it right because that one simple sentence is not rational. Anyone who believes it is, is obviously irrational. The ends cannot be divorced from the means. One cannot create utopia by means of terror. It is impossible and therefore irrational to believe that it is possible.
Your supposedly rational evil people demonstrate their irrationality and their evil by believing that “the ends justify the means”.
But that assumes Stalin's goal was utopia. All the evidence I see is that his goal was power. By that standard, he did pretty well with the mass-murder philosophy and it was entirely rational. "Kill whomever opposes you or whom you suspect might oppose you" worked for Stalin for pretty much his whole adult life.