That one mistake was his undoing.

他一失足即成千古恨。

His lack of experience may prove to be his undoing.

缺乏经验可能就是他失败的原因。

By stopping smoking you are undoing most of the damage smoking has caused.

通过戒烟，你就是在消除吸烟所致的大多数危害。

But alcohol was to be his undoing.

但酒精注定是他的祸根。

Now the bad times may be his undoing.

现在坏时代可能被他终止。