What magic would Harry use to kill anybody?

[Tonia Chu]

No: HPFGUIDX 129173

This might have been discussed before but I couldn't find the thread.
 

I was reading POA again and noticed in the scene after Sirius Black 
dragged Ron into the Womping Willow and to the Shrieking Shack, Harry 
had a chance to kill Sirius.  

"...Harry raised the wand. Now was the moment to do it. Now was the 
moment to avenge his mother and father. He was going to kill Black.
He had to kill Black. This was his chance..."

But exactly what Kind of magic Harry, a 3rd year student, would use
to kill a full grown wizard?  Sure the school hasn't been teaching
kids spells to kill people, right?  And sure Harry wouldn't think the 
wand is just like a knife that you can use it to stab somebody.  

Any thoughts? Thanks!

- Tonia