Let me sum all this up:
1. Colossus couldn't fastball special Leech, because Leech would have cancelled out Piotr's powers, thus no super-duper strength, thus no fastball special.
2. The cure vials were all destroyed, as was the building, the cars, and most of the island. Everyone on the island was incinerated except Wolverine. Even if Wolvie had a vial in his pocket, it would have been destroyed before he could get to Jean. He barely made it as it was. His claws, on the other hand, are INDESTRUCTIBLE. Jean couldn't incinerate them.
3. Leech's power only works within about 5' or less. Colossus couldn't throw him that far without powers, and even if he had powers, he couldn't do it. He didn't make it that far with Wolverine, anyway. Wolvie had to walk the last 25-40 feet.
4. Jean ASKED Logan to kill her SEVERAL TIMES. This was always the plan. Storm asked him much earlier if he was prepared to do what he had to do... Remember? They had already decided that Jean was buried beneath a personality that was too dangerous to live, and too powerful to take any chances. She had (apparently) killed Prof. X, Cyclops, and by the end, about 5,000 other people. She was beyond redemption. She didn't want to live with the knowledge that she had killed so many people-- especially Charles and Scott-- and she BEGGED for release.