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)
you call them bitefull
belligerent
a traveller...
a geologist
Pyromaniac.
geek
a traveller...
sharp
There is no special name for a person who likes squirrels.
How about a "gun enthusiast"?
a troublemaker?
blademaniac