"Who'd been silly enough to build in a spell for Avada Kedavra that could only be cast using hatred?"

I’m not sure exactly what Harry was thinking, but if it simply means that you must “call up feelings of hate” as suggested below, then it might simply be intended as a simple safeguard. Presumably almost anyone can call up such feelings if they tried, but it wouldn’t happen by accident unless you really hate someone. (Given the apparent age of the spell and its character, I don’t think its creator would worry much about accidentally killing someone you hate, if it even occurred to them.)