For the sake of theory, let's pretend these three things that are accurate in the game's context:
The shadow swords are essentially sentient. They have a pretty strong desire to absorb auras, shown by how the Phantom Slasher uses Jeruh's own self-deception and impossible quest for Serena to make "Rhue" into a glorified killing machine. Ghost Horror goes far enough to even choose it's next master [note: I don't know if all the shadow swords do that. Maybe each has it's own method to try and create a sustainable kill cycle? As in Phantom Slasher has Dream Estrana to organize auras and Ghost Horror freely chooses who wields it. Night Reaper also seems to have it's own world inside of it, seen in the Traziun/Kalmar conversation after Traziun kills him]. The idea that the Phantom Slasher seems to have it's own separate identity from Rhue furthers this, especially consider how it tries to keep Rhue in Dream Estrana with deception.
Slade somehow loses either his entire or a large portion of his aura when Phantom Slasher gets lodged within him. The two theories that support this involve how Slade becomes mute in Dream Estrana and how Slade seems to completely change after the encounter. Slade shows shades of self-doubt in short glimpses during the game but after the last encounter with Rhue he becomes inconsolable in self-pity and suicidal. Jopaga said that as shadow swords absorb auras and the dominant aura of the holder begins to lose power, the wielder usually cannot deal with the change and kills himself. Perhaps Slade losing his aura to Phantom Slasher (or at least a large portion of it), losing I guess what the aura represents (seemingly his "perspective") leads to his suicide?
"Rhue" is either the Mimic or Venge or both. This theory is pretty popular, but I'm going to assume it's the version of the theory that assumes that Mimic/Venge is without their own aura (which is generally the idea of the Mimic) and that's why Rhue does not suffer from the negative side effects of wielding a shadow sword (namely, suicide). I like this theory because it meshes well with the shadow swords being sentient; if both of these are true, doesn't the relationship between "Rhue" and the Phantom Slasher seem much more symbiotic? "Rhue" provides the Phantom Slasher a method to murder and absorb auras while Phantom Slasher gives "Rhue" a purpose using the auras it's collected. The Phantom Slasher's lie about being Serena also makes a good deal of sense, too, considering "Rhue" is purposeless without his hunt for Serena and therefore the concept of Serena protects "Rhue" even if it's all just an excuse for bloodshed. The Phantom Slasher in itself seems pretty fond of Rhue, wanting to protect him from the Truth in Dream Estrana as well as always returning to him (specifically after getting stuck in Slade).
Humor me and assume that all three of these are true.
Now, is it any surprise that Ghost Horror chooses Slade as it's next wielder? Slade essentially becomes the perfect puppet for a shadow sword just as "Rhue", a creature lacking an aura to guide it; if Ghost Horror were actually to be wielded by Slade as Ghost Horror seemed to intend, what would have stopped Slade from becoming the next Phantom Slasher? All he would need to do is kill someone with a purpose that can be twisted into a killing motive, have that aura become dominant and you have an identical situation. Perhaps this is why Kalmar has never faced the drawbacks of using a shadow sword. It's possible he went through something similar and has the dominant aura of whatever led or leads the Blana Sera, which seems to be a great dominant aura for any sword that wishes for blood.
Luckily and unluckily (for her), Lyrra gets her hands on the sword before Slade can and something different unfolds. I think the best evidence for Ghost Horror wanting to use Slade as it's puppet is shown in how Ghost Horror chooses Slade while Slade is in the mental state he is in. Assuming that Ghost Horror is out of the picture (as it is in the game, when Lyrra takes it), Slade does in fact kill himself. Now, are we supposed to assume through the general wisdom of the shadow swords that if Slade had picked up Ghost Horror, Slade would have been given a purpose and therefore continued to live (to perhaps kills Lyrra as she ran in, and then maybe go running after Strata with Lyrra's aura dominant, and then essentially killing and becoming Strata to plain ol' kill people out of jerkiness). It would seem rather foolish for a shadow sword to choose a man who's going to kill himself anyway, so I figure there has to be more going on there.
Agree? Disagree? Find a gaping hole in my theory? Been said before? Let me know.