Here's an extensive list of Spidey's kills from Spiderfan.org:
http://www.spiderfan.org/faq/killed.html
Aloha,
This is why it's so important for people to do their homework before they make pseudo-authoritative statements.
Good job Diespinne
Humans Spider-Man Has Killed
ASM Annual #5 Deliberately killed the Finisher by returning a fired missile back to his tank.
Amazing Spider-Man #121 Accidentally killed Gwen Stacy, snapping her neck while she fell towards her death, thrown by Norman Osborn. It was a lose-lose scenario, so Norman gets the credit for the kill.
Marvel Team-Up #31 Killed Drom the Backwards Man. Spidey and Iron fist take on Drom, and smash his mirror on him. This causes Drom to rapidly reverse-age into nothing, effectively killing him. In the narration, Spidey regrets doing this.
Spider-Man vs. Wolverine #1 Unintentionally killed Charlemagne (Charlie). She committed suicide by attacking Spider-Man during his fight with Wolverine.
Web of Spider-Man #91 Kills Whisper-3, member of the Foreigner's Death Squad. Uses Whisper-3 to block Pulse-2's energy attack. Pulse-2 then aborted the mission and commited suicide. Note that the Death Squad was founded on the assumption that the agents were expendable, and would be replaced, e.g. by Whisper-3 and Pulse-3. Even so, Spidey could have simply dodged the energy blast, and showed no remorse for the death.
Ultimate MTU #1 Spidey plasters webbing all over the face of an bystander who makes an anti-mutant comment. The webbing is clearly sufficient to suffocate them. To be fair, I suspect that this is simply a scripting mistake, or else that Spidey deliberately created not-very-sticky webbing, or some other reason why this one shouldn't really be counted.
Amazing Spider-Man Vol.2 #42 Apparently killed Shade while in the Astral Plane, by tossing him against his own energy-storage cocoon, hoping to cause a loophole.
Amazing Spider-Man Vol.2 #50 Accidentally killed two members of the Latverian Liberation Front. Diverted a missile shot by one of their partners to the only place it wouldn't cause any casualties, a hole in the ceiling. Unbeknownst to him, the two guys were arriving to join the fray, and decided to use the hole to enter. Lose-lose scenario, not his fault.
Sentient Robots Spider-Man Has Killed
Avengers #11 Killed sentient Spider-Man android created by Kang.
Spectacular Spider-Man #68 Killed sentient Mendel Stromm android.
Spectacular Scarlet Spider #4 Spider-Man Ben Reilly killed the Master Programmer, a computer version of Doc Ock's mind.
Sentient Creatures Spider-Man Has KilledAmazing Spider-Man #104 Killed Gog, an incredibly powerful quasi-human alien creature in the Savage Land. Spider-Man could not overcome the scaly-skinned behemoth, who was strong enough to kill a Tyranosaurus Rex. The web-slinger traps the creature in quicksand, an option which sickens him, but to which he can see no alternative.
Of course, Gog has subsequently reappeared in an issue of Astonishing Tales, and as a member of the Sinister Six in Spider-Man #18-23 (or was that an alternate time-line Gog). Anyhow, perhaps he's not dead. Still, Spidey did deliberately attempt to kill him, and believed that he had done so, so maybe this still counts.
Hooky: Graphic Novel Assisted Spindrifter in killing Tordenkakerlakk, a magical construct that Spindrifter was fated do destroy. He deliberately put himself at risk so she could do it. The creature was created with the sole purpose of dying so that Spindrifter could mature.
Amazing Spider-Man V.2 #54 Killed Digger. Knowing that Digger would fall apart if he didn't rest, Spidey relentlessly pursued a fight until his opponent died. Spidey's reasoning was that Digger was already dead in the first place, being a gamma-zombie.
Sentient Beings Spider-Man Has Attempted to KillAmazing Spider-Man #122 Tried to kill Norman Osborn over the murder of Gwen Stacy, came to his senses.
Amazing Spider-Man #348 Tried to kill Carnage's symbiote with a sonic burst. It survived.
Peter Parker: Spider-Man #75 Tried to kill Norman Osborn, over the murder of Ben Reilly and his daughter, Norman survived.
Spidey rules with fans who do their research