WIthout going into specifics about the individual, let's say someone's baseline personality is very low on the "happiness quotient" scale. Events such as skydiving, one night stands with FIT models, speedboat racing, climbing the world's top mountains, etc only elevate this to a level of "stasis" (i.e. baseline for the normal human) During these activities, he is "at peace" with himself and the world.
But these "distractions from the abyss" are ephemeral in nature (and getting less effective, and causing him to fear what other recourse(s) he has in the future). For daily, routine activities in his normal environment, he is emotionless and does not speak unless absolutely necessary. People nearby often ask if he is "okay" or "not feeling well" or "is something wrong?" "Why are you even alive, taking up my oxygen?" he thinks to himself, without a verbally answering the concerned party.
Of course, if the means (deception) are necessary for the end (money), he is capable of completely transforming into "someone else" for a short amount of time. The amount of mental energy that's expended for this brief "acting" to appear "normal" is tremendous, often causing him more fatigue than some of the greatest physical human feats known to man.
It's only when this person hears/sees/is told of the failure/pain/extreme suffering of others, that he brightens up and can manage to genuinely be happy. He isn't politically/ideologically/religiously partsian (choosing to have an agnostic view of the world sans "good" or "bad", rather one of only "efficacy" or "failure"). Acts of humans towards other humans dubbed "unspeakable" and "heinous" are downright humourous to him.
There is only 1 caveat. He cannot allow for the suffering of animals. In jest(?), he often jokes that "should I [he] have been a hitman", it must be absolutely a fact the target has no pets. His biggest fear was an animal not knowing his owner would never return home.
He has no emotional bond with other humans, regardless of relation (by blood or otherwise). Humans are the scourge of the earth to him, and he is genuinely confused as to why genetic modification, genocide, and/or selective (homonoid) breeding is frowned upon. "Less people, less problems."
What (non-illegal) careers would covet such traits?