Stalking your ex-lover and killing his family pet is an example of behavior that would be described as psycho.
An obsessed stalker who threatens to kill his ex-lover is an example of someone who is psycho.
An example of psycho used as a prefix is in the word "psychology," which means the study of the mind.
Origin of psycho-from Classical Greek psych?, breath, spirit, soul: see psyche
Origin of psycho-Greek psūkho- soul, life from psūkhē ; see bhes- in Indo-European roots.
(comparative more psycho, superlative most psycho)