I am on my second read of Harry Potter now... finished Half-Blood Prince the other day... still need to read Deathly Hallows.
Severus Snape, in H-BP chapter 28, claims to be the inventor of the spell Sectumsempra. We saw Harry use it on Draco with drastic results. At the moment Harry cast Sectumsempra, Draco was attempting the Cruciatus spell, one of the three unforgiveable spells.
The three unforgiveable spells are Avada Kedavra (the killing spell), Cruciatus (the infliction of pain), and Imperius Curse (the usurpation of another's free will).
How does Sectumsempra stack up against these three? It may not be as quick as Avada Kedavra (unless it was aimed to chop off the head), but it appears that it can be equally deadly.
It seems to me that if the Ministry of Magic knew of Sectumsempra, they'd add it to the list of unforgiveable spells. Snape actually developed an unforgiveable spell while still a student. I don't know the history of the other three spells, but I get the impression that they'd been around quite a while. We never are told that either Voldemort or Grindewald developed as nasty a spell for inflicting physical damage.
I saw the following quote on page 5 of this thread...
Yeah, but Wormtail isn't "bad" by any standard definition; he just became frightened and made the wrong choices. So, by nature he's a coward, not an evil man.
Not bad? By any standard definition? He told a homicidal maniac where his friends hid in mortal fear, knowing full well what Voldemort would do when he found them. He covered up this betrayal by murdering twelve people and framing Sirius for their deaths. Instead of throwing himself upon the mercy of Albus, Harry, Sirius, Remus, or the Ministry, he again chose to assist Voldemort first, in kidnapping and torturing Bertha Borkins and then disposing of her body; second, in assisting Voldemort to return to England; third, in disposing of the body of Frank Bryce; fourth, in murdering Cedric Diggory; fifth, in despoiling Tom Riddle's remains, maiming himself, and spilling Harry's blood in a vile ceremony to restore Voldemort to physical health.
Legal definition? He aided and abetted a mass murderer. He committed kidnapping, torture, and murder.
Social definition? He betrayed his best friend, his best friend's wife, and their child. He was unrepentant of this treachery and actively aided the murderer a dozen years later to try and finally murder that child.
All in all, Pettigrew murdered thirteen people that we know of.
No doubt Pettigrew is a coward, but to excuse his crimes by claiming him to be faint of heart is ridiculous.
Pettigrew is a great foil for Snape. Both were given hard choices. Both assisted a psychopath on his killing spree. But Snape was sickened to the point of repentence. He lived his life to attempt to amend for his crimes. Pettigrew never showed the slightest remorse... oh, he was sorry that Lupin and Black caught him, but in that scene we never see him apologize to Harry.
Perhaps Snape was never truly remorseful for James Potter's death, but Lily's devastated him. That event propelled him from a Death Eater into a member of the Order of the Phoenix. He sought redemption... absolution, if you will... for the rest of his life. Snape is a complicated character, but I doubt the Dementors could have been harder on Snape than he was with himself over Lily's murder. I think he punished himself frightfully... along with blaming James. Pettigrew only sought to escape punishment by seeking Voldemort's protection.
On another note, I've not seen anyone discuss Snape's views of Harry as what might have been for his life if he'd never called Lily a mudblood. Did Snape view Harry as only James' son? Did he see Lily's eyes? Did he ever think "Harry should have been my son?"
Other thoughts.... Snape is a true master of potions. Potion making is not like animal handling, as much art as rules. Potion making is not like transfiguration or charms, they are quick spells. Potion making is a slow process of blending precise ingredients. I think potions helped make Snape the perfect double agent. As Voldemort's agent, Dumbledore's double agent, and pretending to be Voldemort's triple agent, Snape could not rush results. He had to be patient and trust to the slow cooking process to yield results. I think this appealed greatly to Snape.
His ingredients were ingratiation to Voldemort, scorn to the Order of the Phoenix, honesty to Dumbledore, passing information back and forth, and to do his best to be disdained by everyone. He earned Lupin's respect while keeping Sirius' contempt. He earned Bellatrix's scorn while keeping Lucius' and Narcissa's trust.
Snape had his own agenda, more so than any of the Order of the Phoenix or any of the Death Eaters. I wonder what his path would have been if Voldemort had murdered Frank and Alice Longbottom instead of the Potters. Alice's death would not have shocked him as Lily's did. Would anything have awoken Snape to Voldemort's absolute selfishness, avarice, and turpitude? I'd like to think there would have been a point that even Snape would not have crossed, but I'm not certain about it.
What if's are difficult to attempt to solve. Snape's agenda was the destruction of Lord Voldemort. Regulus Black came to his senses and tried to stop the Dark Lord. We don't know if Snape would have followed Black eventually or followed Bartimius Crouch Jr. in serving Voldemort to the end.
For good or ill, Severus Snape's path was laid out when he repented of assisting Voldemort and threw himself upon Dumbledore's mercy. Pettigrew would have been wise to do the same.