ootp, Snape questions, and crucio question

[xpectopatronum]

No: HPFGUIDX 62441

Okay,
I have to first announce that Snape is my favourite character in the
series, so that you understand where my dissapointment stems from.
I've already mentioned in other posts how disgusted I am at how Snape
was treated my James Potter and Sirius Black. It's nothing but extreme
bullying. He was simply minding his own business,and they provoked and
attakced him, three to one (if you include Wormtails verbal taunts)...
Anyway, the two things that I don't really understand are:
1) if Snape knew so much about the Dark Arts when he first arrived at
Hogwarts, how come he couldn't defend himself better from their attack
in the fifth year? I'd really like to hear everyone's thoughts on this.
2) Why does Harry blame Sirius's death on Snape at the end of the
book, when Dumbledore clearly explained Snape's role to him? I
understand that he is grieving, but its not logical to blame Snape,
becuase Harry himself knows very well how stubborn Sirius is. He left
the house against Dumbledore's orders.

Okay, now for the 'crucio' question...
What does everyone think about Harry using Crucio? He really does seem
to be heading down a very dark path. He was angry at the beginning of
the book, and at the end of the book, he seems to hate everything
around him. It's a bit scary, considering how powerfull Dumbledore has
already said that he is.

Looking forward to your answers!
Cindy