Don't each of us have a breaking point? I mean, when Bill gave up a life with Steffy to be with Katie; it was to be with a wife that wanted him. But only a couple of months later, that very same wife starts to show signs of paranoia, depression and shuts Bill out - THEN tries to set him up to be with her sister! There is something seriously wrong with a person that wants to screw up everyones lives because she herself is miserable. Such a typical Logan, if it's not 100% the way that I want things then screw everyone. Being with Brook may have been Bill's breaking point. He had just lost Steffy - who seems to have moved on, and for what? A wife that doesn't want him?
I think Bill did have a nervous breakdown and lost all sensibility when he decided to be with Brook.... finally snapping out of it??? possibly??? and going back to a time where he felt the most clear, totally himself and alive and that is with Steffy. If it weren't for the fact that Steffy is looking at him scared, I would say that Bill is exactly who Steffy wanted back in the day and same man is before her now.
Remember, she lit that match, and now she's gotta watch it burn.