BtVS: Becoming II / What bothers me

What bothers me is this:

Angelus: The blood on my hands must be my own. I am the key that will open up the door

And a little cut on the hand seems to do the trick!

Whistler: Angel's the key. His blood will open the door to Hell. Acathla opens his big mouth, creates a vortex. Then only Angel's blood will close it.

So why couldn't a little cut on the hand have closed it again? And why did Buffy assume that she had to kill him?