Sirius in Shrieking Shack

[Julie Balfour]

No: HPFGUIDX 23887

I have just finished reading the
fantastic FAQs on the Lexicon
(finally!) and they are truly
fantastic - well done to all
involved. 
Normally I am a bit of a lurker,
but something has been bugging me
for a while and I didn't see it
mentioned anywhere yet - if it has
been discussed please point me to
the archives!
In the Shrieking Shack, when Harry
thinks that Sirius was the murderer
of his parents, it seem to imply
that he intends to kill Sirius (he
raises his wand as he want to
kill,but then Crookshanks jumps
onto Sirius) - but how exactly does
he intend to do this? There seems
to be great tension in the scene -
as if Harry actually killing
someone with magic is a
possibility! He has not even heard
of Avada Kedavra at this point, so
I don't see what we are supposed to
think he is going to do,exactly...
Any thoughts/opinions on this?