I agree with Ben that the expression should be "leave her alone". Perhaps you confused it with "let her be"? That sounds slightly formal.
You might say "Just leave me alone!!!" if you're angry and irritated. If you put that in a full sentence, I think it still carries that "annoyance".
I'd say the first sentence reads more smoothly, with the "tone" you intended to give. "I thought it was a good idea just to leave her alone."