Because the positive benefits, or the negative consequences are objective. Someone's personal opinion on the matter is irrelevant to what is objectively beneficial or harmful.
For example, if I believed with all my heart that drinking poison was a benefit to myself, that's not going to stop the objective reality that the poison will kill me, which is clearly harmful to myself.
Helping people overcome psychological problems, is clearly a benefit to the people he is helping. Likewise, killing millions is objectively harmful to the people who are being killed.
That's consequential ethics. What are the consequences of a given act? Based on that, we can judge Stalin to be evil, whereas Larnievc is good (or is at least carrying out a good act in this situation, as we don't know much else about him).