The first think one has to ask about this rabbit is the teeth. Lagomorphs, like their cousins the rodents, have large, evergrowing teeth, which allow them to tear through tough plant matter. However, unlike rodents, rabbits also have a second, smaller pair of incisors in the front of the mouth, which help to reinforce the first pair. The first pair of incisors will become our killing tools for the rabbit. The incisors will become hypertrophied, forming long, spear-like fangs.
But what of the other four incisors? What becomes of them? Well, these teeth become sharp too, though not the saber-like killing tools of the first pair. Instead, these teeth move, the upper pair of incisors stay where they are, and the rear pair move out to the sides of the main two. What's the point of this? Well, quite simply, these rear incisors keep the front pair sharp. To make sure the rabbit is able to kill at any time, the rear pair of incisors grind against the front pair. When the upper pair grow too long, they break off from the pressure given to them by the upper pair, thus keeping them sharp.
As for the rest of the teeth, the only thing I can think of is that the first two pairs of molars change. The first premolar becomes buzzsaw-like and sharp, much like the premolars of the carnivorous macropod Propleopus. The tooth behind it becomes the carnassial, working to shear off flesh from the carcass of the dead. The skull of Carnilagus has some differences from that of its kin. The most noticable is that the condyles have shifted back, and a saggital crest has developed. This gives the rabbit a more powerful bite needed to take down prey.
As for the external appearance of the rabbit, it has a few extra tricks up its sleeve. Its eyes have become larger and...may I say....cuter, in order to take in more light to hunt in lower light conditions, such as at dusk. Its fur is covered in a sort of oil that keeps blood and pieces of flesh from sticking to it, and keeps its fur white and looking clean.
So how exactly does Carnilagus live? My guess would be that due to its small size, the majority of Carnilagus' diet is made up of small animals; rodents and birds. However, its ferocious weaponry "with big nasty teeth" means that if threatened, it can take on foes even multiple times its size, including blundering knights.
...unfortunately, all of these wonderous adaptations do little in the face of The Holy Hand Grenade.