There are literally infinite numbers of ways to end it. The writers chose to write themselves into a situation where Superman had to kill to stop Zod. It's not like the writers said "oh no, he has to kill now! No way around it!" There were plenty of ways. They chose to take the "Supes must kill" route.

The movie foreshadowed very blatantly that Superman would have to kill him. The movie then puts itself willingly in a position where Superman will have to kill. Simple as that.

If you're the type of person who thinks Supes shouldn't kill, it's not that you object to Superman making that decision in that moment. You object to the writers putting their character in that position, which could've been easily avoided in millions of ways had the writers wanted to.

Though I'm generally in the "Supes doesn't kill" camp, I wouldn't have had any problem with that change had I been emotionally invested in Supes or Zod. Since I wasn't, it didn't have the impact I wanted for me, personally. The Vader NOOOOO didn't help either.