Angel of Massacre

Elfen Lied Elsewhere Fic about a sixteen year old male diclonius named Michael Mordare, the first diclonius born in the United Sates of America. The story is set two years after the events of Elfen Lied, with the occasional Shout-Out to its events. Michael Mordare has been raised by his human parents, completely unaware of his mutant powers. Then suddenly his family comes under attack by a US Diclonius facility, leaving Michael as the only survivor. Michael swears to avenge his family, thus starting his journey into the darkness. He also develops a sadistic Split Personality who encourages Michael to Kill All Humans as part of an Evil Plan. Meanwhile, the facility puts Noah Brimstrome, in charge of Michael's capture.

Tropes used in Angel of Massacre include:

Black Bug Room: The theater where the Boy from the Theater presides. Michael gets dragged there when he witnesses his father's brutal murder and when he tries to fight the Boy from the Theater face-to-face. See below.
Batman Gambit: Noah Brimstrome's original plan involved detailed psyche evaluations of Michael Mordare. When Michael stopped following these psyche evaluations, Noah was forced to turn to Xanatos Speed Chess.
Battle in the Center of the Mind: When Michael loses his resolve, The Boy in the Theater beats some into him. The fight takes place inside the theater and is pitifully short. It's one of the rare instances where the host loses, yet still keeps control.
Curb Stomp Battle: The fight between the Boy from the Theater and Michael. Made especially brutal when the Boy delivers an actual curb-stomp.
Enemy Within: The Boy in the Theater later renamed as Samael is pretty much this. What his plans are and what he hopes to accomplish hasn't been directly stated. There's a scene after the Battle in the Center of the Mind where Samael manages to tear off some of the bandages that bind him when he couldn't do this before.
Infant Immortality: Averted so hard with Michael's unborn baby sister. Michael digs her corpse out of his mother's dead body and holds her to his chest. Counts as a Tear Jerker
Let No Crisis Go to Waste: Jeremiah practically quotes this trope when he indirectly accuses Noah of orchestration Michael's escape. Noah's inner monologue reveals that everything is going All According to Plan.
Magnificent Bastard: Noah Brimstrome and Jeremiah Fontenot seem to have a rivalry on who is the bigger Magnificent Bastard.
Mind Rape: Samael drops a bomb on Michael by suggesting that Michael himself is a split personality born from the trauma of watching his father die.
Slasher Smile: The Boy in the Theater displays this in spades at the prospect of massacre. Michael gets in on this in the beginning.
Spanner in the Works: Noah directly quotes this trope when trying to plan around Michael's unpredictable killing spree.
Split Personality Takeover: Subverted when the Boy from the Theater willingly returns control of Michael's body to the rightful owner. Doesn't mean he isn't intending to take it over completely, now is just not the time.
Tranquil Fury: When Michael gets sick and tired of being angry all the time, he resorts to this.