What does the old man's beating heart in The Tell-Tale Heart symbolize?

The true meaning of the beating heart beneath the floorboard is two fold. Firstly there was a slight touch of madness, however the majority of it was guilt for the old murder of the old man.

But the narrator comes in 8 nights and watches the old man, but on the dead of the hour on the 8th night, the narrator kills the old man. :) :p :D
guilt