Osborn may have some argument to innocence under the insanity plea as it is well known that the formula that made him the Green Goblin did cause bouts of insanity (While I don't want to stray away from the origininal incident, William DeFoe's talking to the mask does capture brilliantly the insanity of the character). That said, it's generally understood by the fact Osborn was wearing a mask that he was taking steps to conceal his identity (one known to Mr. Parker at this time) and thus was aware of what he was doing was wrong (thus Insanity will likely not work. This is Marvel NYC, not Gotham, D.C. Insanity defense is more difficult, even if breaking out of prison is just as easy as breaking out of Arkham Asylum in Gotham.).
Mr. Osborn would undoubtedly be guilty of Felony Murder, that is First Degree Murder for an unintended death that was a result of a crime, here kidnapping. Because Mr. Osborn was clearly engaged in kidnapping by taking Gwen Stacy to the Brooklyn Bridge pillar against her will, as well as trespassing as the top of the bridge was not open to the public at any time of day, let alone at night. Notably with Felony Murder, I do not have to prove Osborn's intent to kill Ms. Stacy... I merely have to show that she died as a result of his crime to prove the murder. Doesn't matter if he was throwing her off, as is depicted in the photo evidence, or he was some how committed to saving her. Even if it was proven that someone else acted in a manner that lead to Gwen Stacy's death. Since this question asks about New-York laws, I will not endevor to answer any questions concerning the "Goblin Glider" as that is a matter for the FAA which is federal jurisdiction, not NYC or NY state matter. Mr. Osborn is guilty of Felony Murder or First Degree Murder, however the state chooses to label the crime, kidnapping, criminal trespass, several counts unlawful detonation of an explosive device and equal amount of distributing contaminated food (one of each for each Pumpkin Bomb lobbed).
Now as is custom for me in these less serious legal questions from fiction, I must give a less than serious answer. Please allow time for me to grab a cigar, get a salt and pepper crew cut, grow out a mustache, and review J.K. Simmions' performance to nail the voice.
If the Green Goblin is guilty of Murder because he was committing a crime and Gwen Stacy died... than that can only mean one thing: A masked vigilante like SPIDER-MAN IS A THREAT AND A MENACE! And I love you for providing me pictures... pictures of Spider-man... killing a poor innocent girl who's only family member died some months just prior! I'll make him reviled! I'll make him Infamous! I'll have everyone in this city calling for him to be hung by his own webbing for The Night Gwen Stacy Died!
Ms. Brant! Get in here! Call the Patent Office and Trademark "The Night Gwen Stacy Died!" I want a quarter everytime I hear someone say it out loud or write it in print... What? What? Who the hell is this Stanley guy? Looks like my janitor?! Well Fire My Janitor! What do you mean he's dead?! Well see if we can't buy the rights from that fart's estate! Better yet, claim that I'm related...! no, your right... no one would think I'm just like this cartoonist hack! Find there asking price NOW! Now where was I?
Oh yes, finally answering Spider-man: Threat or Menace once and for all. Parker! I want pictures! Pictures of Spider-Man!
Wooh... Went Meta there. On the question of Parker's culpability in this, there are a lot of arguments both for and against this argument. Yes, vigilantism is a crime, but it's very difficult to prosecute. Spider-man does save people and Gwen isn't the first person to die because of his fights with villains (her father died several issues earlier in a fight between the Web-Head and Doc Oct). However, Spider-Man could make a self-defense plea that has merit. Self-Defense does allow for the defense of others, and Goblin was engaged in a crime against someone when Parker moved to act. That he failed to do so does not factor as this isn't a Good Samaritan issue (and even then, he's the inventor of the webshooter, so should be the formost authority in their operation and stuff like this has occured safely before.). What's more, in the momment, Osborn a noted scientist said that Gwen was dead before the webline caught her and died "from the shock of the realization of the fall." This may be intended to show the Goblin accepting sole responsibility for killing Gwen Stacy. This may not legally clear Parker, but it's enough to so doubt in the eyes of a jury (as comic fans have been debating what was the killing blow to Gwen Stacy for the better part of 50 years and official documents have at various times and printings attributed Gwen's Death to the Fall (Goblin Killed her) and the whiplash (Spidey killed her). This one is less clear.
Perhaps most tragically, Parker would definately not face a civil "Wrongful Death Suit" for a technicality reason alone. Gwen Stacy at this point in the comics had no known living family (her only known relative was Captain George Stacy, who was killed in the aformention fight by Doctor Octopus causing debris to fall towards a child, who Stacy gave his life to shield from the crushing impact.). There is likely no one else in the world who has standing to bring a Wrongful Death Suit against Parker for the incident. Or Osborn for the matter. I'm not sure if a murderer like Osborne can be sued for wrongful death for the same murder.
Edit: The point of the J. Jonah Jamerson bit was to have a little fun with the idea that Peter Parker could be given a sentance of felony murder for his vigilante work leading to a death. I'm not sure if he qualifies however because the U.S. does allow for Self-Defense as a defense to actions as well as citizens arrest (attempting to stop and detain a criminal they witness committing a crime while not a sworn law officer in so far as you will turn over the suspect to law officers as soon as possible and are reasonable in the application of force.). Additionally, this would only work if we ignore the immediate next issue in the comics, which continued with Green Goblin's attack and ended with Goby impailing himself on his own glider. While the Green Goblin was elevated over Doc Ock as THE Spider-man villain, Norman Osborn remained "dead" for 22 years in real time, which is quite a long time given that comicbook characters are notoriously easy to kill but difficult to keep killed for a lengthy stretch of time.
