"Psychopath" and "sociopath", for the matter, aren't clinical labels to begin with and, on top of that, Kizaru isn't enoughly explored as a character nor is Oda exact with his usage of psychology (e.g. the Vinsmoke siblings were stated to be emotionless yet showed Ekman's emotions), so my short thought:
Oda is portraying Kizaru as a person who actually enjoys executing instead of doing it just as a job (hence why he asked Sengoku if it was okay to kill the enemies in Marineford and was about to directly pierce Oars Jr's, I think, head with a laser to murder him) and has a bit of a childish mentality (in a way I see him as a "twisted monkey", a playful yet evil character; the way he blew up that tree in Shabaody because he overdid his kick, or captured many pirates because of his tantrum after failing to catch the Straw Hats, or feared the Celestial Dragon's scolding like a kid who did something wrong... Even last chapter with his panel entertaining Sentomaru, whenever I look at Kizaru I see an evil kid). He also plays a facade since it's obvious he's smarter than he looks (connected to the science division and the only admiral shown studying as a kid).
Oda doesn't care enough for Kizaru to be a psychopath, and don't believe most of the people posting in this thread since they don't know what they're talking about (as I said, psychopathy isn't a clinical label in DSM). To put it simple, Kizaru seems to be a person who plays himself dumber than he truly is but also has low emotional control and seeks sadistic excitement under a cover of professionalism (for example, I'm sure he'd enjoy murdering Sentomaru at this point). As I said, he's a monkey in a twisted way.
Call it however you want.