Anger leads to Hate, Hate leads to suffering.

The Narn had a long history of hating the Centauri Republic for the brutal way the Centauri occupied the Narn homeworld. Formerly a world carpeted in forests, jungles and lakes, the planet was transformed into a wasteland with only a patch here and there too small to do any significant good for the planet's ecology thanks to the Centauri.

Narn is an extremely resource-rich planet in the galaxy, and the Centauri took advantage of the primitive Narns in a foul way. Many Centauri noble Houses systematically strip-mined the planet of its resources to build ships and guns, but for some of the natural treasures of Narn, many plants such as the sacred G'Quon Eth plants, which are used for religious ceremonies, found their way into Centauri cuisine. Disdain for Narn religious and cultural beliefs, the increasingly bad dust pollution, the murders, genocides, and slavery resulted in many of the Narn using their anger to fuel their resistance, but the Narns took it very far when they travelled to other worlds and enslaved worlds in much the same way the Centauri had done to them.

When the Narns drove the Centauri away although there were many factors which went into the withdrawal of the occupation, their rage and hatred didn't escape them, and while it did do significant strides in helping the reconstruction of the Narn's culture, the hatred was unleashed on the galaxy when they achieved interstellar travel after claiming pieces of Centauri technology.

The Narns didn't see the irony when they started launching attacks and stealing planets from their inhabitants, decimating whole eco-systems, taking resources and using them to build the increasingly hungry Narn war machine with the long-term plan of beating the Centauri and punishing them for what they had done, and while their hatred helped them become a major interstellar power, they caused a great deal of suffering for many races who had barely achieved space travel.

The Narns learnt their lessons well from their former masters.

War, Death, and Pain - Sandra Garibaldi, 2450 AD

Author's Note - As Master Yoda says in Episode 1, fear is the path to the dark side and the Centauri stoked hated and anger into the Narns which spread to both of their worlds.