When Harry meets Black (PoA)

[aaronmjfisher]

No: HPFGUIDX 31844

I'm puzzled by two things in this chapter (Cat, Rat & Dog):

SPOILER ALERT!

1. When Harry, Ron and Hermione end up at the Shrieking Shack and 
Harry has his first encounter with Black face-to-face (as a human), 
why, in the ensuing struggle, does Black appear to try to strangle 
Harry to death, when really the person he's talking about killing is 
actually Scabbers/Pettigrew, whom he knows is protected by an 
unknowing Ron?

Was Black merely trying to push Harry aside by his throat, in order 
to stop Harry kicking and punching him? I just find this a very 
strange thing for Harry's godfather to do to him...

2. With what spell is Harry actually going to kill Black with? As far 
as I know, he hasn't been taught any lethal spells or incantations at 
this point, and he's only told about the Unforgivable Curses 
(especially Avada Kedavra) in the next book, GoF.

I can only assume that Harry was going to turn Black into something 
else, like a small rodent, and then stomp on him - but then, I'm not 
sure if Harry's been taught to do that either at this point...

Any ideas, people?