They don't.

If Green Goblin is going to kill anyone, it should be Harry.

Norman Osborn is such a shit-bird that he tested the Goblin Serum on his son Harry first.

When he finally snaps during Dark Reign, Norman takes the serum as a last-ditch effort to evade capture and kidnaps a one-month pregnant Mary-Jane Watson-Parker (first and only time she's ever taken hostage.) Goblin carries her to a bridge and threatens to rip her in half unless Spider-Man shows up unmasked.

Peter Parker shows up, unmasked and tells Goblin that he did everything that he asked.

Green Goblin: You did, didn't you? (Throws Mary Jane off the bridge.)

Spider-Man leaps to grab her, but Goblin grabs him by the neck and slams him into the ground, forcing him to watch as his wife plunges into the river below.

However, Harry, having injected himself with the serum and grabbed a prototype glider, manages to rescue Mary-Jane.

After dropping her off safely, Harry joins the fight and the two Goblins engage in an epic dogfight. Tragically, Harry's heart, by some combination of stress, drug abuse, adrenalin, and as an unfortunate side effect of the serum, gives out, and Goblin beats his own son to death.

Harry's death infuriates Spider-Man and he goes completely berserk, beating Goblin's head in until his hands are soaked in yellow blood. The force of the blows causes the section of bridge beneath them to collapse. Spider-Man grabs the ledge, while Goblin grabs his leg.

Goblin's weight is too much, and Spider-Man loses his leg below the knee. Determined not to allow Goblin to escape into the rive below, he tries to snag Goblin's leg with a web, but accidentally snaps Osborn's neck.

Crippled, his identity revealed, and a father, Peter Parker retires from being Spider-Man.

You know, just a highlight reel of every shitty thing Norman Osborn has ever done (Ultimate Goblin killing Harry, 616 Goblin throwing Peter's loved one off of a bridge, MC2 Goblin costing Peter his leg and trying to kill Mary-Jane's unborn child.)