There are several approaches.
Tolkien's Orcs are born of evil and incapable of intentional good. Thus, slaughtering them is justifiable.
Meanwhile, the D&D Orcs merely have a propensity for evil; it is wired in but able to be overcome.
It's variable by setting whether or not the "evil races" were made irredeemable, made bad but redeemable, prone to bad but overcomable, or entirely free will based but with a violent culture...
If you really want to teach lessons via play, have the setting say one thing but the reality be the other...
This reminds me of the tale of fraz'urblu.
Once a mighty duke of a layer of hell (currently dispaters layer) he attempted to usurp azmodeus.
Dispater beat him in the mist embarassing fashion and fraz fled to the abyss to avoid az's punishment (to date, being the only rebelling duke to avoid horrible punishment for any openly attempted usurpation with the exception of glasya. But thats because glasya is az's precious girl and the princess of hell obviously got off easy.)
When in the abyss fraz teemed up with grazz't, orcus, and demogorgon to defeat the queen of chaos (the og queen of chaos. The obyrith not tiamat.). When the battle was over they each took an equal portion of her essence.
When they did this, everyone but fraz got much stronger (hed been pretending to be a demond prince). Fraz though, got weaker because he was taking on chaotic essence on a cosmically fundemental level. Seeing that fraz got weaker via this dilution outed his true nature (as a being encapsulating fundemental ideas of lawful evil) to his allies and he had to flee again.
As you can see he was able to change but due to being such a purely LE being to the point of being a being of conceptual forms (see platonic philosophy) it actually harmed his very being to change. Because he became less like his true self.
For races (or other groupings of beings) whos connection to fundemental concepts are deep enough an interesting way to show this innate difficulty of going against their wiring while also showing ita possible (and thus never giving a chance to change with most groups of beings is perhaps not necessarily rational) can be that if an example being tries to change in signifficant fashion you could show that while difficult its possible while maintaining the idea of just how difficult it is by showing that its hard on their very body and soul.
Perhaps a nymph who turns evil starts being unable to heal people to the point that even if she wants to she cant help but accidentally give terminal illnesses to those she tries to heal.
Maybe a red dragon who turns goof starts to lose a little muscle mass (because he no longer is able to indulge his appetites fir flesh beyond what he needs to live) and hus scales start to be slightly shiny and translucent around the edges (resembling raspberry glass) because his body is still chromatic but none the less is taking on slightly metalic dragon qualities superficially because hes turning good.
Maybe a vampire after centuries of practice looks a littke sickly but has managed to reduce how frequently he needs blood to once every 3 months by slowly supplementing his fedings with more and more raw meat along with the blood. As a result he doesnt have fast heeling unless hes fed in the last week and its night time but he none the less doesnt require as much feeding.