L. Dutch has a perfectly solid answer, but I feel it needs a bit more meat. So looking at your last question as well, I think the key is that your species needs to be sprint-type and ambush predators, but they may be losing a motivation to still be tool users. claws tend to be at odds with tool use, since the idea is that the claws compensate in animals for a lack of tools. They certainly can evolve anything with enough pressure, and claws can be extremely useful.
Finger nails can certainly evolve into claws. They grow thicker, wider, and more deeply buried in the hands. The hands may become less flexible and the finger shorter to give better strength to those claws, but reducing the nimbleness that aids in making and using tools. Getting the claws to be extremely sharp might be a bit of a stretch evolutionarily, but this isn't likely to be a huge problem if your elves aren't hunting big prey.
If they are, they still likely need to be using tools (at least knives, clubs and rocks, but spears are amazingly useful). Hominids traditionally used pack tactics and endurance hunting to repetitively injure large prey and wear them down. Their injuries in doing so it meant Neanderthals tended to have injuries similar to rodeo performers, and for your slight species this would be bad.
- Claws can assist in climbing, so your elves may be able to nimbly pursue prey up trees. This also allows their fragile selves to flee up those same trees.
- Digitigrade legs give higher boosts of speed, but may in full bipeds tend to cause tumbling and require lots of traction to use effectively. Having longer front limbs that can be used for running may help, but the claws allow greater dig into soft materials like wood or even dirt, so they will be less likely to both fall OR run in soft soil and churn up the dirt due to lack of traction.
- Claws may assist in digging like a badger, so prey that flees down burrows can be efficiently dug out.
All these behaviors would tend to lend themselves to a species evolving into a niche similar to a cat or other small predator, pursuing smaller prey rather than tracking larger. Pure carnivores are probably not doing much gathering, and are less likely to be developing things like agriculture. They may as a species be evolving away from the sapiens model of smarter tool users, and instead be "devolving" by human standards into a species where tool use is increasingly less important. The growth of a mane fits well with a species that increasingly doesn't use clothes to thermoregulate. Humans are arguably partly domesticated and increasingly social, so your species is less domesticated in behavior than modern humans. The cannibalistic behavior they exhibit fits well with this idea.