Disclaimer: I'm not muscling in on JK's turf - just gambolling on it, like a spring lamb, having fun working out the literary and psychological puzzles which she is having fun setting us.
This is a filk (Science Fiction or Fantasy-based folk song) rather than a poem as such, so to get the full effect you need to mentally set it to music. It goes to the traditional Irish tune My Johnny was a Shoemaker, which can be found on the Steeleye Span album Hark! The Village Wait.
I play at bad to serve the good
'Twixt the shadows and the light;
The tyrant's terrors I've withstood
In the strongholds of the night,
For to break the evil I once swore to serve
On the path that no-one sees,
The narrow path that no-one sees.
In bitter anger I was raised,
In sorrow I was schooled;
Forever scorned and never praised,
By bitterness I'm ruled.
My wits and wit will armour me from harm
And my tongue shall be my sword,
My cutting tongue shall be my sword.
The comrades who think me their friend,
It's them that I'll betray,
While those whose lives I would defend
Think me their enemy;
And the fools I teach will never understand
That my scorn is half a jest,
Bitter jest -
Who have I left to share a jest?
I slew the friend who saved my life
To serve his grand design,
I go in grief and bitter strife
To the exile's death that's mine.
Now every man's hand's raised to strike me down,
And who will mourn for me?
Who is there that will mourn for me?
I don't actually think Snape is going to get killed in Book 7 (I certainly hope not) - but I think he probably thinks he is.
I can't paste an URL in here because ffn would delete it, but readers who wish to hear the tune for this song should Google on "My Johnny was a Shoemaker" and "Mudcat Café", which will take them to a folk-music site which has different versions of the tune, and then select the one marked "tune as modified by Gay Woods and Maddy Prior." This is not exactly the same as the one on Hark! The Village Wait but it's close enough to give you the right idea.