Why would you assume gorillas are moral agents?
I haven't assumed this, if people want to deny facts that's their prerogative, but I don't have to indulge such idiocy.
Again just survival instinct says do what others find acceptable. That means if others find killing accepted, roll with it.
That would still require the ability to differentiate between right and wrong behaviours, which by definition is morality. Are you saying killing is always immoral? You may want to read the bible then, as the deity you imagine is real is depicted as a genocidal mass murderer for most of it. Were it real, it would also be by far the most prolific abolitionist.
It is simply a fact that all animals, including human animals, that have evolved to live in societal groups have evolved the ability to be moral. This does not mean all their moral judgments are the same, since morality is subjective. You seem to be conflating the ability to be moral, with adhering to what you personally consider moral.