Its not that sort of altruism.
We are speaking here of philosophies, not the messy empirical reality of the world as it actually is.
And a Godless altruism is indeed likely to become inhumanly vicious. As we have seen. Anything is justifiable for the good of the masses, including, possibly, killing off most of them.
A heresy can be a terrible thing indeed.
Then it is not actually altruistic. Nothing about communism really points towards the well-being of others, but towards its dogma. It claims to have the well-being of the “proletariat”, its imaginary superior class of humanity, in mind, but even who they identify as “proletariat” cannot live in a world without family, private property, religion and national borders.