The way we perceive evil has been biased greatly by how it is presented in stories, tales and in mass culture today.
There is this one villain, who is responsible for everything, and we know he dies in the end, but for the most part of the show, we are more interested in what the villain does. Way more adrenaline, it's all spectacular fights and explosions, and we already know that, as the credits roll, all the evil is absolved by the fact of the hero killing the villain.
In real life, a death of a dictator, a tyrant or a murderer, even if it happens in our lifetime, doesn't magically undo the harm caused.
Did it help that Joseph Goebbels killed himself, his wife and six innocent children with cyanide ? As a symbolic event, perhaps, but it certainly didn't bring back the millions of the Jews, nor did it ease the suffering of their families.
Did it help that Joseph Stalin died in a puddle of his own urine, presumably because he was feared and hated ? Symbolically, perhaps, but it certainly didn't stop communism from marching on for decades, nor did it prevent his monuments from being erected to this day.
Great evil has a life of its own, and it doesn't have a single face.
Consider what happens when a primordial tribe masks up and performs a war dance. Even though every single individual has his own part in what's going to happen, the masks they wear let them distance from the guilt and the blame. It's their God, or the Leader who is to blame. If something goes wrong, they figuratively speaking tie the ritual stone to the Leader's neck and sink him. By doing so, everybody is freed from the responsibility.
As a sign of conscience, we must face the consequences of our own actions and bear our part of the guilt, not hide behind the Führer's figure.
And we should not expect that evil disappears when a dictator dies.