"I didn't need to" is just fine (for American usage). Also, "I didn't have to" has the same meaning. I didn't need to go to the library because none of my books was due, but I wanted to see if one I've been waiting for has come in.
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AnonymousHuh? I don't understand what you're saying.
In that sentence I meant that saving his life wan't necessary and I didn't save his life becasue I knew about it but because it was just a coincidence that I didn't even tried to rescue him