I still think that, like Zack not healing himself after he kind of died even though he could have and then letting Kate heal him, this situation was one half on purpose.
He probably addicted people by accident, but he made Kate feel a lot more than others on purpose, too.
In my opinion that was a challenge Zack set. He clearly stated before that Lisa was destined to be alone unless he did something about it, because by herself she wouldn't find anyone. Zack worked in the background there and set her up with Libby's aunt (? I think it was her aunt...one of her relatives in any case) in a way nobody really noticed.
So I think Zack wanted Kate to develop something in her personality and left solving the problem - and addressing it - up to her on purpose. Though the initial addiction was an accident. How was he or anyone else supposed to know what would happen if he tried some....advanced Alede tricks and feeding techniques?
In my opinion he did it because in the second book she's a Zack worshipper and extremely similar personality wise so Zack feared that, if he let things go on she would become an exact copy of him personality wise aside from the fact that she's Alede. So my theory: like sending her away from him, to where she couldn't just follow his actions and had to stand on her own he set this one as a challenge for Kate to go her own way and, that's the most important part, AGAINST ZACK.
Later in the book Zack held back on purpose and simply supported Kate in what she was doing.
Doing things that way other nations saw that Kate was truly his equal as a Line walker Plus they got to realize that her healing powers were far above anything else out there. Meaning, the Line walkers became a group of people powerful enough to rival any other nation in potential. By adding that one twin and his cousins the group suddenly became one that was growing quickly and had to be controlled in some way. Follow that line of thinking and people were actually happy that they formed a nation - meaning that nation had to make sure none of them suddenly started assassinating people in closed and guarded rooms or something. Because a handful of nearly godlike people going rogue without restraint is pretty damn scary. After all, they couldn't force all of the line walkers to marry a dozen people each to control them like they did with Zack alone.
So, yeah, I think Zack did most of what he did on purpose. Except the addicting thing, that happened in book 2 and he didn't realize it.
