Waylander (The Drenai Books) by David Gemmell

Waylander is an amoral Assassin and has been getting steadily worse over the Twenty years since he started hunting down the killers of his family. This cumulates when he assassinates the Drenai king starting a war that seems to be leading to the utter destruction of the Drenai nation. Until one day he saves a young priest Dardalion and is infected by his purity. Their association causes them to change Dardalion loses some of his purity and Waylander finds himself acting for good for the first time in years. When he is visited by the king’s father he promises to find the Drenai treasure The Armour of Bronze to make up for his deeds. Can he find and return the armour before the Drenai are defeated.

This is one of my favourite books and some of Gemmell's best work. Waylander is a fantastic character and his struggle to survive after being infected with good is really entertaining.