Evidence against it being the Killing Curse:

• The Killing Curse doesn't shed blood; the target simply drops dead.
• The Killing Curse is characterized by a flare of green light of a very particular frequency, which consistently appears in descriptions of its use.
• The Killing Curse is two words, not one.

That said, we don't know what incantation or ritual is involved in making a horcrux. It may very well be blood magic, and use merely the word "Horcrux!"