'Want You Gone' is the ending theme for Portal 2 and I think it really suits Shadow and who he is. Anyway, you should check it out on YouTube or something.

"Well, here we are again
It's always such a pleasure
Remember when you tried to kill me twice?
Oh, how we laughed and laughed
Except I wasn't laughing
Under the circumstances I've been shockingly nice

You want your freedom, take it
That's what I'm counting on
I used to want you dead but
Now I only want you gone

She was a lot like you
(Maybe not quite as heavy)
Now little Caroline is in here too
One day they woke me up
So I can live forever
It's such a shame the same will never happen to you

You've got your short, sad life ahead
That's what I'm counting on
I'll let you get right to it
Now I only want you gone

Goodbye, my only friend
Oh, did you think I meant you?
That would be funny if it weren't so sad
Well you have been replaced
I don't need anyone now
When I delete you maybe I'll stop feeling so bad

Go make some new disaster
That's what I'm counting on
You're someone else's problem
Now I only want you gone
Now I only want you gone
Now I only want you gone
Now I only want you gone"