prompted nightmare

First off, unless it is your style, I suggest using capital letters for words you may want to emphasise, or for the first word of each line. Or capitalize only one word. For example "Mercy".
When a word is capitalized it gives it more power, you must use that to your advantage. Mercy seems to be the theme of your poem, and yet you have "prompted nightmare" for the title. I think it's a little irrelevant and should be changed. the best poetry are ones of ambiguity, your seems to just be plane to you, and hard to understand by others.
This is just me, but I belive your opinion on mercy is wrong. Mercy is defined as "compassion or forgiveness shown toward someone whom it is within one's power to punish or harm." Because of this, when you say "mercy is not always kind" I find it frustrating. Because mercy is never deserved. Because forgiveness is also never deserved, it can't be bought or earned. Its only given by one's heart from care. (But that last bit is my opinion)

In conclusion, use capitalization, be careful on your choice of words and applying those words (such as mercy and saying it is not always kind), and try to use language that can be interpreted in more then one way for your audience. (ambiguity).

I hope this helps. Good luck in your poetry writing.