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 can be read as either a filk (Science Fiction or Fantasy-based folk song) or a poem. It exists in two versions, a concise and a long one.
The concise version is, I think, more satisfactory if read as poetry, but if you want to take it as a filk then the only tune which I could find which fit it was A Policeman's Lot is Not a Happy One, which really doesn't have the right sort of feel to it.
The longer version fits best to the tune of the traditional Border ballad The Lammas Tide, but it can also be sung to the tune of We Poor Labouring Men, which can be found on the Steeleye Span album Bedlam Born.
If I punish son for father then I'm cruel,
But if Hagrid does the same then that's just cool.
If Black still sneers and spites me he's a hero;
If I do the same to him then I'm a fool.
If I chide the dim or lazy I'm the harsh one;
If Minerva does the same it's just good fun.
If Moody isn't pretty that's just quirky,
But my face and hair are seen as cause to shun.
If I do the old man's bidding then I'm evil;
But if Potter feeds him poison at his will
He's a saint, but I'm a villain for obeying:
The scape-goat driven out for all to kill.
If I should blame the father's sins upon the son I'm cruel;
When Hagrid does the same thing, in him you think it's cool.
If Black still baits and sneers at me like schoolboys, that's OK,
But if I do the same to him you think that I'm a fool.
If I chide the dim and lazy you think me the callous one;
Minerva does the same and you think that it's good fun.
Though Moody's face is ugly you think it has some charm,
But my appearance is a curse and seen as cause to shun.
You think that I am evil since I did the old man's will,
But Potter fed him poison and made him drink his fill;
And both of us obeyed him, but only one is blamed,
So he's your saint, while I'm sent out to be your scapegoat still.
Hagrid, of course, afflicted a cowering, terrified eleven-year-old child with a painful and humiliating pig's tail just because he didn't like the kid's father - and OK, we know that Dudley was a nasty little bully who probably deserved it, but Hagrid didn't know that. He just knew that Vernon is a loudmouth.
Minerva McGonagall punished Neville for his chronic bad memory by deliberately humiliating him, making him sit outside in the Fat Lady's corridor and have to ask for somebody to let him in - and we're told that it was done specifically as punishment. It wasn't just a matter of not trusting him with the password, because in that case she could have told the prefects to make sure to look out for him and let him in. Snape, although he is rather overbearing and nags the boy too much, never actually punishes him for the vagueness and poor memory which he cannot help.
Snape bears a grudge against Sirius, OK, but he bears the grudge because young!Sirius tried to murder him for kicks, and because adult!Sirius still boasts about the murder-attempt and says that his victim deserved it - and even so, he tries to save Sirius by telling him to stay away from the Department of Mysteries. Sirius, on the other hand, bears a grudge against Snape so powerful that it leads him to claim that young!Snape had deserved to be murdered; and that grudge isn't because of anything which adult!Snape has done, because when Sirius says that the murder-attempt had served young!Snape right, he hadn't even met adult!Snape yet. By his own admission, Sirius's grudge against Snape is that he was an irritating, nosy boy who followed the Marauders around. Sirius's grudge, therefore, is both more vicious than Snape's and much more petty.
Yet Hagrid, Minerva and Sirius are considered to be lovely, kind people, while Snape - who has an unfortunate, defensive, snarly manner but actually behaves rather better than any of them - is portrayed as the epitome of evil.
This poem is actually a companion-piece to an essay called But Snape is just nasty, right? which you can find on my website at w w w . whitehound . co . uk / Fanfic / (just take out the spaces). A writer called seomensnowlocke posted (on ffn, which is really against the rules) an essay called Why Snape Does What He Does which irritated me so much that I felt impelled to write a complete rebuttal, and this little filk emerged as a spin-off.