How to correctly translate 为达目的不择手段 in English?
Generally speaking, I encourage you to check out the meaning in Chinese when you encounter 成语. It took me a while to finally get to know that it is hard to understand what a given 成语 means in English but is clear enough once I read the explanation in Chinese (this holds true of any words but especially so in the case of 成语, which is closely related to the culture).
Back to the topic, 不择手段 means:

指为达到某种目的，什么样的手段都可以使出来。

In English, the word means:

In order to achieve a goal, a person uses whatever he or she could (including anything considered immoral or dishonest by others).

Don't try to understand it by an English dictionary as the listed meaning is quite awkward and hard to grasp. For example, in the case of 不择手段:


unscrupulously

by fair means or foul



These explanations sound less redundant, but seems to me the loss of original meaning.