And I would have been a sitting duck.

我就会成为一个坐以待毙的人。

Goliath is a sitting duck. He doesn't have a chance.

哥利亚基本就是坐以待毙，他没有机会出手。

You don't want to be a sitting duck when the shooter gets here.

枪手来时，你可不想成为显眼目标。

The rest of it means nothing except that if he knew she was dead he knew he was a sitting duck for you.

其余的一切都毫无意义， 只是如果他知道她已经死了， 他就知道他对你来说是个活生生的鸭子。

And, that is an expression that we use for those sorts of situations, someone is a sitting duck when they are going to be hurt or something bad is going to happen to them because they are without any protection.

而且，这是我们在这种情况下使用的一种表达方式，当某人将要受到伤害或一些不好的事情将要发生在他们身上时， 他们就是一个坐着的鸭子， 因为他们没有任何保护。