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I'm used to translating English auxiliary "must" with a Latin gerundive: hic necandus est "this man must be killed".

But what if I want to say "this man must not be killed"? I would read non necandus est as "it's not necessary to kill him", which is a somewhat different meaning (it's ambivalent about whether he should be killed or not).

In my experience many languages confuse lack of desire and desire of the contrary.
For example, I would like to be able to say "I don't want coffee" as the negation of "I want coffee", meaning that I don't have a desire to have coffee.
To say that I am actively against drinking coffee, I would prefer to say "I want not to have coffee".
But, unfortunately, English doesn't work this way, and "I don't want coffee" is construed as "I want not to have coffee" instead of the more ambivalent reading.

Similarly, the Latin non necandus est is more literally "it is not necessary to kill him" but could also be read as "it is necessary not to kill him".
I found examples of similar constructions, but it is not easy to decide which meaning is intended in each case.
I would consider both readings valid in general.

I see a couple of ways to express "it is necessary to not kill him" without ambiguity:

1. Take a new verb with the opposite meaning: servandus est
   


2. Explain in more words: necesse est eum non necare
   


3. Work it into the structure of a sentence: curandum est ne necetur
   


4. In some cases you might be able to use a negative order: noli(te) eum necare
   
   (There are also passive imperatives.)

There are three or four impersonal verbs to express what is appropriate, or legal, or obligatory.

1 děcet, it is appropriate
2 dēděcet, it is inapproptiate, unseemly.

Ut nobis decet; As seems right to us.
Oratorem irasci minime decet, simulare non dēděcet. It is not professional for an orator to get angry, it is not unprofessional to pretend (to get angry). Cicero Tusc., 4,25

Non nos decet necare; ‘It is not right for us to kill.’
Dedecet necare; ‘It is unseemly (uncouth? it is not very nice?) to kill.’

3 Lĭcet, it is lawful

(cf. illĭcĭtē, adv. illegally; illĭcĭtus adj., illegal)

Lĭcet nemini peccare, Cicero Tusc., 5,19 'Nobody is permitted to do evil.'

So, Licet nemini eum necare. 'It is not lawful to kill him.'

4 Oportet, it is a duty, one ought.

Est aliquid, quod non oporteat, etiam si licet; quicquid vero non licet, certe non oportet.

'There is something which one ought not to do, even if it is legal; but anything illegal, certainly ought not to be done.' Cicero.

Certe oportet non eum necare, 'Undoubtedly, one ought not to kill him.'