YOU SAID:

If they will not obey then they must die

INTO JAPANESE

彼らが従わないなら、彼らは死ななければならない

BACK INTO ENGLISH

If they do not obey, they must die

INTO JAPANESE

彼らが従わなければ、彼らは死ななければならない

BACK INTO ENGLISH

If they do not obey, they must die