Maybe it's not necessary for Zack to physically kill them himself, but to perhaps feel as if he did? Like, say, said friend dies protecting him and thus he feels he's at fault for the death?
Heee. Because nothing can stop the Zack. He is a force of nature. *nods wisely*
Hrm, I can see Itachi and Zack not meeting for a while, considering how large the Uchiha clan likely is/was. So that'd give them a few more years. Especially if Zack were ignored by the rest of the clan, due to the fact that he "hasn't manifested the Sharingan". Despite the fact that he, you know, likely has and just hasn't told the clan that.
Yeah. Likely got used to being around a Legendary Sanin due to his Sensei being the personal student of said Sanin.
Heh. I always pictured Turks as being the equivalent of the hunter-nin, while Soldier would equate to ANBU. *grins*