The Xon weren't any more evil or good than the race that survived; two
equally sentient species emerged on the Centauri homeworld over the course of
evolution. They were more or less separated by distance and land masses and
oceans, so there was little contact until either side developed the technology
required to find the other. It was the next best thing to a first contact
situation, except it's on their own world; they were very much alien to one
another. Somewhere along the line, it was decided that this world wasn't big
enough for both of them, so they began a campaign of slaughter against each
other. Londo's people won.

jms