"Nihilism" can certainly mean different things. Western nihilism might indeed presuppose self-interest and a resentful rejection of an inhuman world. But the basic point is the recognition of reality's moral emptiness. Buddhists, too, talk about the emptiness of all things: they're empty of any essence that corresponds to a simplistic conception--and that includes any moral conception. Hence, the enlightened Buddhist should be unattached to moral desires, which makes morality (compassion, selflessness, etc) vacuous. That amounts to the essence of nihilism.