Well, yes, that's a bit of a given. Except that for me, it's both : the toxic sides of the relationship in S6 are equally due to Buffy feeling like shit and using Spike as result AND Spike being ultimately soulless and thus not being able to tell right from wrong, hurting Buffy (sometimes deliberately, sometimes not) as a result.I would say the unhealthiness of Spuffy stems from Spike's obsessive nature and lack of soul. It would have ended in the same place, even if Buffy hadn't died. Spike almost shot Buffy with a shot gun in S5.
It's part of what I find fascinating and compelling about S6 Spuffy : it got really dark (with some light touches here and there), but at least it was equally dark from and on both ends.