sicko 
/ˈsɪkoʊ/ noun
plural sick·os /ˈsɪkoʊ/ noun [count] informal : a person who does disturbing things that a person who is mentally normal would not do
▪ She was afraid to walk alone for fear some sicko might be lurking in the shadows.
▪ She was afraid to walk alone for fear some sicko might be lurking in the shadows.
