How would Harry have killed Sirius in PoA?

[hedge2staxy <hodgins@ssi.net>]

No: HPFGUIDX 49708

Here we are--that suspenseful moment in Chapter "Cat, Rat, and Dog" 
of PoA.  Harry has recovered his wand in a scuffle and is now 
preparing to end Sirius Black's life.  A combination of Crookshanks 
and Lupin, stop us from seeing if Harry would have killed him, but 
then I started to wonder.  How?

I guess if you layered enough hexes onto a person you could kill him 
or the easy Snape way out would be to immobilize Black and let the 
dementors do the rest.  Somehow, though, neither of these meshes with 
the spirit of the scene and as a 3rd year at Hogwarts, Harry really 
wouldn't have had the sophistication to do anything more than this, 
would he?  

Any theories as to what might have been on the tip of Harry's 
tongue?  Perhaps, we're not supposed to think this far, and the 
moment is just to convey the depth of Harry's anguish at the loss of 
his parents.