Joran is just too crazy. In Equilibrium he seemed like a mostly normal guy that had violent impulses that he couldn't control and one day he broke and murdered someone. In this episode he's completely psychotic and that's the core of his entire being. How did a guy like that ever pass any sort of psychological evaluation? It just doesn't make sense and it's not as interesting as my previous understanding of the character as a normal guy that's a bit unhinged.
I acknowledge that this might be more fodder for the "Trill symbiosis is poorly explained" complaint, but I have a possible theory for that. Perhaps it's because we're not seeing Joran as his truly realized self in an objective manner, but rather the Joran that Jadzia must confront to achieve full equilibrium? Which would be in keeping with the supposed point of the rite. So while it is indeed the "true" Joran it's also a distorted reflection of him as well, with certain of his traits ascendant and others suppressed. Same guy, but different arrangement? I mean, even if we accept the premise that the hosts' personality is indeed "stored" accurately along with memory and experience (which of course we must for this episode to make sense), that "pattern" is operating as filtered through, or interacting with, two or three wholly different brains, nervous systems and personalities. Physiologically and spiritually, Jadzia isn't Joran and nor is Sisko. Seeing as his memories are new to her, Jadzia presumably has less experience in incorporating Joran into herself compared to other hosts, and those aspects of him which are most disturbing and difficult would be the remorseless killer. So perhaps the Joran she projects is a Joran whose personal aspects are weighted towards what she still needs to confront? So Sisko has Joran, but only parts of him, and the true balance and ratios of the character aren't in evidence. Is the zhian'tara a full transplant, or just a transfer of those aspects the current host could do with understanding better? And I guess Sisko's nervous system might be having trouble too, and only certain facets of the personality pattern are getting through - maybe Sisko subconsciously rejects hosting the remorseless killer parts, and so in turn "projects" them outward?