People are not good or evil, and I am not a Moral Relativist.

Good vs evil does exist, but people aren't inherently good or evil. Those that do evil lack the capacity to see why good is preferred over evil. They lack the ability to discern what actions are more likely to be good and which are more likely to be evil through rational thought.

No one in our lifetime will know what is truly good and what is truly evil, but a rational thinker must use their faculties of reason to progress towards good. Can you not see the absurdity of seeing someone as evil simply because they were born with a brain genetically weaker or developed with an inability to reason?

When you encounter a squirrel with rabies, should you think the squirrel itself is evil? No, it is suffering from an ailment that is causing it to act irrationally.

Since people aren’t inherently evil, and it is not their fault they do evil, how should we treat them from a moral perspective?

This is where justice as a virtue comes in. Justice is necessary to maximize utility for all. The rabid squirrel will never recover and should be put out of its misery. The serial killing human must be separated from the rest of the population, although shouldn’t be dehumanized. Prisons, in their current form, are excessively inhumane, but the underlying separation from society is still important to protect others. The fellow law-abiding citizen should be reasoned with but don’t fool yourself into believing you can convince someone to gain agency. They will always be a follower of the public thought leaders, whoever they may be.

Never allow yourself to judge a person too harshly, always humanize them. Remember to be critical of their actions and philosophy, not their irrational worldview. Never pretend their suffering doesn't exist.