Psychopaths are violent by nature, they act impulsively, they are short-tempered. They refuse to accept the consequences of their actions, often blaming others (Pop had Genco; look what I got). The don't learn from their mistakes (just as Sonny never learned to curb his temper), they don't benefit from negative feedback and they can't stop their impulses.
He gave Tom a lot of shit, but you never got the sense that Sonny would murder Tom. Shoot, after he said what he said to Tom, he quickly took it back and said he didn't mean it (even if he did). Apologizing shows empathy, don't you think? Sonny was impulsive, sure, but he showed a LOT of empathy - he showed it when Michael was leaving to meet Sollozzo. Sonny had a lot of love for his family, and I don't think he'd ever whack anyone like Michael did.
While I never personally thought of Santino as a 'psychopath', SB is justified in that definition.
Think about it, not just the boffing of Lucy at his sister's wedding, the hit on Bruno Tattaglia (after no murder committed and Michael being slugged by a cop), not listening to Tom's reasoning on 'the war' the day Vito returns home, the merciless beating of Carlo in defense of his sister...and finally rushing to his ultimate demise on the causeway...all pretty much indicate the same mindset.
Had he not been hit when he was it was only a matter of time and he may have caused alot of damage, leading to a much longer war in the meantime.
