Uncanny Web-Slinger
Super Freak
In the Under The Red Hood animated movie, jason Todd blames Batman for the state of Gotham and all the lives the Joker and other villians have taken over the years because Batman never ended the cycle by killing the villians.
If Zod was sent to the Phantom Zone again, he'd escape again and attack Earth again and kill more people again.
Superman ended the cycle by ending Zod. Its not a "Superhero" thing to do, but Zod isn't coming back to kill people and be locked away where he can break out and kill again.
Granted he didn't do it for that reason but it works out better for potential future victims of Zod.
If Zod was sent to the Phantom Zone again, he'd escape again and attack Earth again and kill more people again.
Superman ended the cycle by ending Zod. Its not a "Superhero" thing to do, but Zod isn't coming back to kill people and be locked away where he can break out and kill again.
Granted he didn't do it for that reason but it works out better for potential future victims of Zod.