How to use なきゃ correctly

Question

よっぽど悪いことさえしなきゃ...
Unless you do a really bad thing...

Is なきゃ a contraction of ないことには? I can find the latter in my text books but not the former.

Weblio suggests that it can mean both "Unless one ..." and "one must ...".

Would I be correct in thinking that the meaning is determined only by the position in the sentence. So, in my example sentence, if there were no following clause would it mean "You must only do bad things."

In summary I'm a bit confused about the usage in general. Any information (in English) would help. Thanks.

Answer 1

First, なきゃ is a contraction of なければ:

• What is the meaning of りゃ in this phrase?
• Meaning of 話になんない

Second, the literal, primary meaning of なきゃ/なけりゃ/なければ is "if not".

なければ often comes at the end of sentences, and that means the following part is omitted. What is omitted depends on the context.

• (Unless ...,) something bad may happen. ⇒ One has to do ...

  これからは、悪いことをしなきゃ（ダメだ）。
  From now on, I have to do bad things. (e.g., after joining Mafia)

• (Unless ...,) the situation would have been better.

  ああ、あんな悪いことをしなきゃ（良かったのに）！
  Oh, if only I didn't do such a bad thing!

• (Unless ...,) you'll be fine.

  よっぽど悪いことさえしなきゃ（問題ない）。
  Unless one does really bad things (, there would be no problem.)

The first one ("one has to...") is a common idiomatic usage of なければ/なけりゃ/なきゃ, and that's why it's listed in dictionaries.

In the sentence in your question, such a sentence almost always means something like "Unless one does really bad things... (he'll be OK)". But it may mean "I have to do even really bad things!" in a very limited situation.