Disclaimer: I don't own Harper's Island, so I claim no responsibility for the plot flaws.
Warning: If you haven't seen the finale yet, this contains the identity of the killer, so read at your own peril.
"Why?" Abby asked as the tears streamed down her face and the glass shard she held cut into her hand. "Why?"
"So we could be together on this island forever," Henry smiled. "It's what you always wanted, isn't it?"
"No, I got that, you sicko, I meant why everything else? Why did you believe Wakefield when he said he was your father? He could have been lying, or simply mistaken. It seems likely for a man driven mad by hatred. And why did all the other characters have to die? You could have let a few more escape, if you really wanted to. Killing everyone you've known since grade school was really unnecessary. And Shane? He would have happily left us alone. And why didn't the state police come after the officers Wakefield killed? Maybe they would accept a few hours of not hearing from them without any suspicion, but a whole night? They would have come without us having to call them. And why didn't they find us? Wouldn't they have searched the entire island thoroughly for any survivors? Isn't this Kelly's house? Why did you take me to Kelly's house? And why did you put ink in her eyes anyway? You didn't have to. And you don't have to claim responsibility for every death that happens on this island, you know. And wouldn't someone else notice that there are people in this house now, when the only occupant died a few days ago? Why did you tell Sully that you put the money in Hunter's boat? It wasn't like he was asking or anything. And I thought Mr. Wellington put the money there, to bribe Hunter to not tell you that he was working for him, to break you up. So it was Uncle Marty's? I never thought the origin of the money was really in question. Why didn't you kill Trish when you two were alone at the inn? Why hand her off to Wakefield if you were just planning to kill her anyway? And if it was really so hard for you to kill Trish, why didn't you just pick some random girl off the streets and seduce her? And what about everyone else on this island? What, were there ten people living on the island to begin with? An island of ten people doesn't need a sheriff with two deputies, or its own newspaper, or a church, which seems to exist just in case two people decided to get married here. Why didn't anyone besides the wedding guests and their local friends see Wakefield? And why didn't anyone try to get help from these invisible locals? Why didn't any of the locals try to go to the Cannery after the massacre there? I get why all your shots at Wakefield missed, but how come when Sully and Danny shot at the door to the passages, with Wakefield behind it, he didn't get shot? There were bullet holes in the door, so the guns weren't loaded with blanks or anything. Why didn't anyone notice that people were missing earlier? Oh, Uncle Marty found a girl, Booth went home to his mother, and Lucy just… went somewhere… without telling anyone… somehow… How improbable! Everyone came to the wedding, but no one planned to actually stay for the ceremony! And they never found Lucy's body, did they? And what happened to Gigi? The last time we saw the poor dog, she was starving in the woods with no caretaker. And how does someone get shot, fall off a cliff into the ocean, and live without anyone being aware of it? Wakefield didn't alter his appearance or anything. Which reminds me, what happened to the doctor? Couldn't someone have contacted the doctor to, say, help Cal when he was shot or something? Or was it when electricity was cut? Wasn't the electricity still out when Trish took her shower? Then how did she get hot water? You can't get hot water without power, and there was steam in that shower. And why are you so obsessed with me anyway? Do you have some creepy incest thing going on?"
With every word exposing another flaw, Henry began to grow smaller and smaller. By the time she had finished, he could conveniently fit beneath her shoe.
"But, Abby!" he stared up at her. "Abby, isn't this what you always wanted?"
"Um… no. Wasn't that obvious from the beginning?" Abby dropped the shard of glass and positioned her foot over Henry.
"No, Abby, don't do it! I'm your brother!" Henry screamed.
"Tough." Abby stepped down.
Splat.
"Killing is hard? Really? I thought that was pretty easy, actually. Maybe I'm really the murderer, and Henry was just a victim," Abby muttered. "It would make about as much sense as anything else on this show."
She went back upstairs to get some more sleep.