Buffy has a boatload of other issues that she has to deal with, so it's still down to Faith and Spike playing Xander's bodyguard. The good news is that they knew it would happen going in. The bad news is they didn't take into account that someone who'd actually been affected by Xander's psycho days might be mulling revenge.
The reasons for Xander breaking Kennedy are...well...complicated. Definitely the issue of control is at play, but there are other issues that are less clear-cut.
The whole point of soulless vamp Xander is that without all his human baggage, he can make use of devious impulses without guilt. Hence, his ability to get to Slayers and Watchers, his ability to attract victims, and his ability to break down Kennedy to the point where she's in a twisted co-dependent relationship with him.
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The reasons for Xander breaking Kennedy are...well...complicated. Definitely the issue of control is at play, but there are other issues that are less clear-cut.
The whole point of soulless vamp Xander is that without all his human baggage, he can make use of devious impulses without guilt. Hence, his ability to get to Slayers and Watchers, his ability to attract victims, and his ability to break down Kennedy to the point where she's in a twisted co-dependent relationship with him.