Warning: Long.
Morality is a rules structure used to maximize the survivability and sustainability of a social entity (eg: a family), while minimizing the energy expenditure (in terms of analyzing a social conflict and determining a proper action, keyed off of all possible rules the creature must adhere to) of the creatures comprising the social entity.
Good and evil are labels with respect to morality. Good is acting in accordance with the moral rules. Evil is acting not just not in line with the moral rules, but in such a way that the social entity suffers. Also, the harm may not be direct or explicit, but only understood after observing actions and their repercussions over time. Optimization of moral rules attempts to minimize the number of rules covering the greatest number of social entities that a creature is a member of, though that can also lead to an oversimplification that harms further development.
Creatures that take actions that harm the social entity while not in positive service of a higher-level entity (eg: the tribe vs the family) are also considered evil, despite not being part of the native social contract. This is largely in service of 'othering' said creatures, so as to amplify conflict-based actions by members of the social entity, and thus improve the social entity's chances of survival. This would be analogous to a body's immune response to a disease.
Note that there are a variety of moral rules structures, each trying to optimize the survival and development of the social entities that they are attached to. However there are some rules that are close to universal because they serve very basic needs in the survivability of most social entities.
From this, we can see that "evil" creatures are 'others' (not part of an aligned in-group) that violate the tier-0 common moral rules of other social entities. In order for evil creatures' social entities to survive, they must have found a survival mechanism that uses these otherwise-avoided interaction rules. This would be analogous to parasites in the animal kingdom which find an evolutionary niche that other creatures have left open.
Note that there is no violation of free will, here. An evil creature (as described above) has found a set of moral rules that benefit it and the social entities that it is a member of, and is adhering to those rules just as any other creature (such as a human with 'normal' moral rules) does. This is likely related to its biology (since different biologies allow for different types of actions), though it's just as possible for the moral rules to have influenced the creature's biology as it is for the biology to have influenced the moral rules.
This all works because social entities are in just as much an evolutionary competition as biological creatures are, and "survival of the fittest" still applies. Social entities will evolve to be well-suited for competition with similar social entities, but that will inevitably leave them ill-suited to compete with certain others. As nature abhors a vacuum, entities to fill those gaps will naturally arise.
So an inherently evil race would be one that can strongly and naturally benefit by taking advantage of the taboos of other races, while also not being terribly well-suited to the normal tier-0 moral rules.
As an example: Goblins have an extremely high birth and maturation rate, while not benefiting so much from long-term parental care (such as for education). There is low value per individual life, so murder is not much of a taboo (though their cowardice might arise because murder is so easy and casual). Their biology is adapted to feeding on raw meat (similar to carnivorous animals), and quickly expanding populations find 'civilized' races (which focus more on child care and education than raw survivability) incredibly easy prey. Thus cannibalism (in the sense of humanoid eating humanoid) can easily become a positive survival trait for the tribes. This can go further if they have traits such as seen in Goblin Slayer.
There's no strong incentive to try to interact with other civilized social entities, particularly as any breach of moral rules would immediately trigger an antagonistic response, which would return things to the original state. (IE: They are in something like a Nash equilibrium, where any change in the choices they make makes things worse if you can't move the entire society to a new moral state in one fell swoop.) For example, if you let goblins into town to try to incentivize them towards using the town shops, and one goblin decides to eat a child for lunch while he's there, the town will quickly reject them rather than figure this is just growing pains for goblin society. And even if goblins were adopted into 'civilized' society, that just leaves an evolutionary gap for another race to fill.
So really, it's not that hard to have inherently evil races when you have a large number of wildly varying intelligent creatures, each trying to carve out its own evolutionary niche. I mean, we already see a little of the overall concept even with the basic races, what with the stereotypical dwarf living under the mountain, or the elf living in the woods. The main thing you'd want to consider when creating such a race, though, is, "What is taboo to this race? What is it that this race considers an evil violation of their most deeply-held precepts?" Because the standard Good/Evil axis is pretty much entirely from the human social perspective.
