A blood stained knife is much more frightening than a clean, unblemished knife as it gives and impression of already injuring or taking the life of another person before it has been turn on you, the victim. This makes the person holding the knife seem more threatening as he has evidentaly already killed or injured someone and will just as relentlessly kill  or hurt you too. 

The curvature of the knife would also feel more painful going in and out of a persons flesh, as it will dig in and embed into flesh or bone before being ripped out again. This knife in question also looks blunt. A blunt knife would be far more painful to be stabbed with than with a sharp knife, as more force is required to piece the skin, as opposed to slicing the flesh.