Back when Duckula was still the benevolent CD version, Goosewing came up with a 'foolproof' plan to end him once and for all: befriending him and making him think Goosewing had truly given up. Over the course of a few weeks, maybe a month, though...Goosewing realizes that CD really ISN'T a threat, but he's already got a group of more competent vampire hunters ready to strike, and if he backs out now they'll just as easily turn on him, so he continues with the plan. Reluctantly leading him into a trap.
This is Duckula's final straw, and what causes his mindset to shift; if the world is only gonna see him as a monster, then he'd be a monster and MAKE them love him.
And Goosewing? He lives out the rest of his days in regret and passing on his knowledge of vampires to others, writing down his experience in a journal.