I agree, evil is a relative term and matter of perspective. Are these men evil? They chose to do evil things. It is too easy to label a character as good or evil but in Shakespeare it is not that easy. All of his characters are human beings struggling with their world and survival. They illustrate what can happen to a human being pushed to the limit by any number of things---lust, greed, power, envy, rage, etc.
Claudius, for example, is a younger brother in a world where younger brothers didn't inherit much. Old Hamlet is referred to as old. How old was Old Hamlet? How old was Gertrude? How old was Claudius? They live in a warlike world where a younger man might be a better king. Is his love and affection for Gertrude real and if so, is that evil? As we all do, Claudius justifies his actions.
Is Iago evil or just somebody consumed with jealousy? When referring to Othello, he tells us that he will inflict him (Otherllo) with the same poison that consumes him. Next thing we know, Othello is jealous. Does Iago feel justified in his actions? Of course he does.
Aaron is an alien in both cultures, Goth and Rome. He has been the victim of hatred and prejudice which has hardened him to other people's suffering. He sees a chance to get his revenge and goes for it.
Shakespeare was a great playwright and gave us a glimpse into the vulnerability of these characters. It would have been too easy to make them one dimensional. The three dimensional versions of characters who do evil are much more interesting and very recognisably human.
All of these characters roll the dice. They set things in motion and once begun they have no control over what will happen. They get what they desire momentarily but lose in the end.
I don't think that makes them evil. They chose their actions and know they must live or die with the consequences of those actions.
Never judge a man until you've walked a mile in his shoes.
This seems like an enumerative essay, which means you qualify a characteristic and then provide specific examples that exemplify how each character fulfills your standards of the term "evil." You might want to delve into some psycho-pathological terms or review Freiderich Neitzche's "Beyond Good and Evil" to help provide the parameters of your argument.
From there, identify categories that provide insight into how you judge evil: motivation, back story/personal history, intention, execution, acknowledgment of the crime, and aftermath. Does the antagonist get what he wants?
Each character is evil in his own way, because each perpetrates treason, murder, perjury, and betrayal. Yet, you have to understand why they commit these crimes.
Claudius is the younger brother who is jealous of his brother's accomplishments, yet he never foresees the repercussions: loss of wife, kingdom, confidante and confidante's family.
Iago is a lover scorned. His BFF, Othello, betrays his trust and forfeits their friendship in order for Othello to gain credibility in choosing Michael Cassio for distinction.
Aaron is the anti-Othello. As a minority, he is aware of the prejudice against him. He is the original "Gangsta" or "No Limit Soldier." He can only gain from his allegiance to Tamora. His depravity is tantamount and acts as the combustible engine that drives Tamora's vengeance against Titus.
Or judge evil by body count!!!!