Not to mention that it would have to be Severus who kills someone in order to break his soul in two so that part of it can go into the Horcrux.
There's more logic for you!
hmmm. Now, what if it had happened beforehand, say at DD's death? Maybe that really did crack Severus' soul and it latched onto something in the Astronomy Tower? The Elder Wand? The Invisibility Cloak? Harry again? A Telescope? Draco? Lots of potential for a Severus Horcrux. :)