"I ate a cat once," Alucard said matter-of-factly.
Abraham hadn't minded; the old man had never cared whether Alucard devoured the mice roaming the house or the stray cats outside – on the rare occasion the vampire was actually allowed to spend time outside. Cats had become a delicacy at some point, while mice and rats were a welcome diversion to the cold, stale blood in plastic bags.
But that one time he found a cat indoors – a young one at that, the taste was telling –, he terribly upset Abraham and young Arthur. Abraham had scolded the vampire; revoked his permission to roam the common rooms on the ground floor. And young Arthur had been crying for days, mourning the cat. At first, Alucard didn't quite understand why the boy was crying over a cat of all things. They had plenty of those in the neighbourhood, after all. The Hellsing estate wouldn't suffer any losses from a cat less, after all, wasn't Alucard their resident cat, albeit oversized and bipedal? He was much more efficient at keeping the household rat- and mice-free.
But then, after the distraught young Arthur had symbolically buried the cat - there was no body to be buried, after all, since Alucard had devoured it whole -, it dawned on the vampire. It wasn't just any cat. It was the young boy's pet companion, the one that kept the boy's bed warm at night and lulled him to sleep with its purr.
After that, Alucard decided that cats were no longer a delicacy. Abraham's wrath and young Arthur's grief had tarnished their taste; made it bitter with the taste of a grave mistake committed.
-- an excerpt from one of the dozen fic drafts on my GDrive
Edit: now available on AO3:
An Archive of Our Own, a project of the Organization for Transformative Works