You could use most of the words in this poem honestly.
Evil can be one because of how strongly you are describing a force. When using evil you are saying someone or something is worse than bad and worse than horrible. Saying someone or something is evil is saying they are wicked and no good can come from it. It is not simply used to say bad but worse than bad. What the author is saying is that something worse than bad lurks in benevolent smiles of torture. He/she is using it to be expressive. They are trying to make a point that bad is not good enough and this force is worse than bad. I hope this helps ;)
