Answer:
I couldn't find which poem you are referring to, but I can help you define this word so you can compare if it fits with its use in the poem.
Explanation:
Something diabolical refers to something evil. It does not always have to be something strictly religious where the word refers to something related to "the devil." Sometimes it can be scary, tenebrous, or evil.
It is something excessively bad or wicked.
This definition should fit the poem you are talking about.
This is from the story of "Civil Peace" and is written by Chinua Achebe. To describe the thieves, they were cocky and bold, but when Jonathan's family began to yell for help, the thieves respond by helping them with that. Hope this answers your question.
I protested my school fees when classes were all remote and we were charged for in person activities even though there was no activities.