Language is complex that people at times, disagree on what words are truly opposite in meaning to other words.
Macabre is defined as disturbing and horrifying because of involvement with or depiction of death and injury.
Benign on other hand, being synonymous to words like gentle, kind and harmless, appears to be an unfitting antonym.
But because of benign's similarity to the word pleasant which apparently an antonym of macabre, according to Thesaurus.com, you can then probably say that they are the opposite of each other.
When working with antonyms, a thesaurus or antonym dictionary can be very helpful in finding the perfect word.
These reference sources will give definitions and parts of speech, has lists of antonyms, synonyms, and sometimes will even show similar sounding words, homophones, rhymes, and will show examples from literature.
Get more tips on how to deal with antonyms at examples.yourdictionary.com.