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.'
5 Opus est, (3) can mean 'must be,' necessarily.