Yes.
There is quite a lot of demonic blood around. So I fully believe Nicholas attacked his attacker back, and left quite a grevious wound on the creature. There's a fair larger amount of the demon's blood around the shop than your parent's own.

Take solace in that. One of his friends is here trying to find a lead with magical means.

As to why it was him... I do not know. It is a question I wish I had the answer to, as well.