It's similar with other traits. Being more aggressive than average might increase your success in life, but if you have too much you will get in trouble.
Another way to look at it is that some traits are harmful for the individual but beneficial for the group. Insomnia does a number on your health, but the tribe wants someone to jump at every cracking of a twig in the night, just in case it's something bad.
I mean, we also call extreme aggressive tendencies a mental illness or disorder.
In general, the standard in mental illnesses is that in order for a diagnosis to be made, the condition must interfere with leading a fulfilling life; a narcissist is only diagnosed with NPD if their narcissism prevents them from achieving their goals in life (hold down a job, maintain relationships, etc).
Another way to look at it is that some traits are harmful for the individual but beneficial for the group. Insomnia does a number on your health, but the tribe wants someone to jump at every cracking of a twig in the night, just in case it's something bad.