Deathstroke was actually fairly average when it came to psychology. What he excelled at was calculating human nature to design traps, usually with the goal of eliminating a target rather than treating patients or helping others solve their troubles.
Much of the time, even when he used psychological tactics in his plans, he merely knew that they worked without knowing why. When it came to true expertise in such techniques, professionals like Harley were the ones to look to.
For example, if someone liked eating shit, Deathstroke would see it, nod, and think, yeah, this person probably had psychological problems.
But why had this person developed pica? Where did the physiological effects and psychological abnormal pleasure of this behavior come from? Could such a fetish be treated? Or did it even need treatment?
Those were things he did not understand, nor did he need to. Even with the omnipresent Quantum Concepts and the Origin Codex in hand, capable of seeing through the origins of all things, he had no reason to study why someone liked eating shit.
It was merely a social phenomenon that existed. As long as it did not block his path, why interfere with someone else's choices? People were interesting precisely because of their diversity, were they not?
So, it was probably something like this.
If Superman saw someone stuffing themselves with shit by the roadside, he would walk over and say, "Sir, you need help. I can take you to see a doctor."
If Deathstroke saw it, though, he would most likely say nothing and walk past as though nothing had happened. If he was in an especially good mood, he might even encourage the shit-eater to stay true to themselves and ignore everyone else's stares. In other words: "Eat whatever you want. Go for the