In these four lines, ONLY change the Bolded word to make the entire poem have a more negative tone.
"And he was always QUIETLY ARRAYED,
And he was always HUMAN when he TALKED;
But still he FLUTTERED pulses when he SAID,
"GOOD MORNING," and he GLITTERED when he WALKED"
Let's help you change the tone by helping you understand the meanings of the words in bold, and thus their synonyms.
Quietly arrayed: this means modestly dressed, not dressed in a flashy way.
Human when he talked: this means he was compassionate and sensitive and unjudgmental when he talked.
Fluttered pulses / said: this means he caused listeners to feel excited and emotionally moved when he spoke.
Good morning: this, of course, is a cordial greeting given during morning-time.
Glittered when he walked: this is clearly a metaphor for someone who shed the aura of good will around himself when he walked.
This is a rather mystical of fantastical tone, rather like a pleasant fairy tale, perhaps Mr. Magorium of Mr. Magorium's Wonder Emporium is called to mind.
To make this a darker tone, you need to think of synonymous words that would fit a dark wizard instead of a cheerful wonder worker.
Someone on the dark side might be bleakly clothed; or speak chillingly; or growl instead of speak; or smoke instead of glitter.
Now that you have background and a few plausible examples (though certainly not the only options!), you will be able to sort this out in good order.
You not only change the tone, you change the meaning. Here are some words to add that make the poem bitter and also change the meaning.
"And he was always completely grumpy,
And he was always angry when he shouted;
But still he skipped pulses when he whispered,
"Get out of my way," and he stomped when he left"
I have tried to depict a slighly scary, negative tone.
By changing the words, we are going to change the rhythm of the poem. I hope that is acceptable. I don't think that you will be able to/need to change all of the words. The most obvious change to me would be to change "fluttered" to "agitated" since they have very similar meanings. You could also say he was always "dull" when he talked or that he was (first line) "always drably dressed."
Litteacher8, the bolded words have to be changed using different SYNONYMS, not Antanoms.