Change the speech:
"If you don't keep quiet I shall shoot you", he said to her in a calm voice.
1
He warned her to shoot if she didn't keep quite calmly.
2
He said calmly that I shall shoot you if you don't be quiet.
3
He warned her calmly that he would shoot her if she didn't keep quiet.
4
Calmly he warned her that be quite or else he will have to shoot her.