How is the tone in "The Tell-Tale Heart" callous and disdainful?

Well, we need to remember that the narrative choice that
Poe made to relate this chilling tale presents us with a profoundly unreliable narrator
who himself expresses a series of emotions and views. Let us remember that he stresses
the way that he loves the old man and that there was no element of revenge or greed in
his desire to kill him. Yet at the same time, you are right in indicating that the way
that the narrator kills the old man, producing terror in him and enjoying this fact,
would contradict this earlier assertion that he loved him, and it definitely gives rise
to a callous tone. Note the following quote when it is clear that the narrator delights
in the terror he causes the old man before killing
him:

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Presently I heard a slight groan, and I knew it
was the groan of mortal terror. It was not a groan of pain or of grief--oh, no!--it was
the low stifled sound that arises from the bottom of the soul when overcharged with
awe... I knew what the old man felt, and pitied him, although I chuckled at
heart.

Note again how we are
presented with this seeming contradiction as the narrator says that he "pitied" his
victim whilst at the same time "chuckling" at the terror he is inducing. It is clear
that the manner of killing creates a callous and disdainful tone, as the narrator could
be likened to a cat playing with a mouse, deliberately terrifying it, before killing his
victim.