In Julius Caesar, where is the evidence that the dictators are calculating and ruthless?

You pose a really excellent question and the entire play hinges on the answer. In fact, the conspirators in Julius Caesar do not actually have much evidence that Caesar is a calculating, ruthless dictator. Brutus says specifically that Caesar is a reasonable man (“I have not known when his affections swayed more than his reason”) and his justifications for killing him hinge primarily on what Caesar might do if given the crown and allowed absolute rule (“How that might change his nature, there’s the question”). It's not at all clear that Caesar is a dangerous or dictatorial man, or is at all inclined to become one. Without this sort of evidence, the entire play becomes far more complicated: Is Brutus truly a hero trying to protect Rome from tyranny, or is he a self-righteous man whose violent plot brings civil war and chaos? It's a question that the play leaves open.