People who enjoy killing are most likely psychopaths but other then that there are no speical words for someone who likes to kill...well I guess you call just call him a ruthless cruel person with no regards of human life a or maybe Misanthropy (a person who hates humans)
A person who enjoys killing is commonly referred to as a "murderer" or a "serial killer." These individuals have a deep-seated psychological issue and often lack empathy towards their victims.
you call them bitefull
belligerent
sharp
geek
a traveller...
Pyromaniac.
a geologist
a traveller...
There is no special name for a person who likes squirrels.
Imaginative
a troublemaker?
How about a "gun enthusiast"?