The final paragraph of the story creates the full impact of its Gothic slant. Readers probably suspect at the end of Part III that Emily intends to murder Homer with the arsenic, although the motive is never completely clear. Then, upon her death in Part V, readers are no doubt surprised and appalled at the description of Homer's body on the bed and the clothing and gifts laid out as if for a wedding night. By that time readers know that Emily was completely insane. They assume she killed him and put his arms into an embrace around her for perhaps a short time, then locked his corpse up in the room as in a tomb.
But the information in the final paragraph takes Emily's madness to a completely different level. Run-of-the-mill lunatics don't sleep with dead bodies for years. It took Emily's hair several years to turn the "iron gray" color that it stayed for the rest of her life. Thus the gray hair on the pillow next to the rotting corpse indicates that Emily slept there for possibly years after Homer's death. Necrophilia is not a common literary subject, and for many people, this is the only story or work of fiction they will have read that touches on necrophilia. This puts Emily in a class by herself as a literary "heroine" or antagonist. Whatever sympathy readers may have had for Emily because of her domination by her father, unpleasant family situation, and mental illness, that sympathy likely dissolves upon discovering the depths of horror to which her oddity led. Where readers may previously have been sympathetic toward Emily because of her nosy neighbors, readers may now wish that the town had inserted itself more forcefully into Emily's life in order to expose the ghoulish behavior that was happening literally under their very noses.
This question is very subjective, meaning that the answer depends on the person answering it. I can't tell you what the effect of the final paragraph is for you, nor can I tell you how it may have changed your attitude toward Emily.
All I can do is tell you my own reaction to the revelation that Emily has been keeping Homer Barron's dead body in an upstairs bedroom for many years: I was horrified not just because the body was there but because there was evidence that she had been sleeping beside the body. After the shock wore off, I wondered, if she loved him (or was stubborn enough not to let him go) so much that she preserved him, why didn't she spend her final moments in the bed beside him? That is, why didn't she lie down and die beside him?