Or...this might be a case the cure being worse than the disease...

But why would Cologne want to do that? From what we've seen, the MPD
is his greatest source of power. Each personality is highly focused
on what it's doing, making it much more competent. 

Of course, Cologne may not realize that killing off the various 
personalities will also destroy all of his special powers. 

We'll have to wait and see. ^_^

At this point I'm not sure what Cologne's -true- motivations
are, although, without the Tribe, she may feel that Ranma will
be easier to deal with...  No female personalities to get in
the way of a wedding.  No super-strength to get in -her- way...

-Anand