I know how good grendels can breed... so this is really handy

Letter A
Afraid
Anxiety
Anhialation
Apocalyps(e)

Letter B
Bloody
Bloodymary

Letter C
Curse


 Letter D
Death
Diablo
Demon
 
Letter E
Evil
El Diablo

Letter F
Frankenstein
Frenk

Letter G
Greed

Letter H
Hell
Hellraiser
Hydra

Letter I   
Incubuss

Letter J
Jack the ripper

 Letter K
Krypton
Kryponite

Letter L
Lurch
Lucifer

Letter M
Monstertje
murder
Molotov
Mancer

 Letter N
Nitro
Necro

Letter O
Opium

Letter P
Plage

Letter Q
Quetzal Coatl

Letter R
Ripper

 Letter S  
Spell
Seth
Slayer (sended in by smsduim)

Letter T
Torture
T-rex
Terror

Letter U
Underground
Usurper
Undieing
Urgh

Letter V 
Violent
Vicious
Vampire

Letter W
Warg

 Letter X
Xeno

Letter Y
Yield

Letter Z
Zion