Like Zayne, he too was considered a failure, but where Zayne learned to deal with his resentment (and suffered partially from indifferent teachers). Haazen blamed others and gave into his spite.
After using the Sith War to murder his employer (while getting horribly injured) he manipulated the man's grieving widow into forming an organization that would supposdly help stop the rise of the sith, but which would actually help facilitate their return.
After 33 years he set his plans into motion by hijacking control of the Republic Fleet and making it bombard coruscant, killing thousands. Zayne was able to convince Haazen's erstwhile apprentice Lucien to turn on Haazen and they were able to successfully trick him into giving Zayne an opening to remove the gauntlet used to control the fleet.
Lucien then hurled Zayne and his friend away and used the guantlet to redirect the fleet's fire onto the estate, killing Haazen and stopping his plot.