After being caught and tortured by the native tigers, he developed a complete hatred for the species. When he was later in the control room of a dam being attacked by the tigers, along with the Doctor and Fitz, he opened the floodgates and drowned the tigers. Regretting his actions afterwards, he stayed on Hitchemus to work with the colonists and tigers to save the planet from impending disaster. He realised, though, he could never fully make up for his actions, and had ruined his relationship with the Doctor. (PROSE: The Year of Intelligent Tigers)
Behind the scenes
In a 2005 Livejournal comment thread, Kate Orman confirmed that while she intended to leave a degree of ambiguity to the Doctor and Karl's relationship, she completely intended the two to be in a romantic and sexual relationship.