When "altruism" is mandated, it ceases to be altruism. It is then slavery. People don't get that often. People who genuinely care about other people and would give of their time, money, etc. on their own often fail to make that connection. I know several people like that.
The definition of altruism varies with what the person using it has as an agenda, and which one is the true form of altruism. To quote Webster:
Main Entry: al·tru·ism
Pronunciation: 'al-tru-"i-z&m
Etymology: French altruisme, from autrui other people, from Old French, oblique case form of autre other, from Latin alter
Date: 1853
1 : unselfish regard for or devotion to the welfare of others
2 : behavior by an animal that is not beneficial to or may be harmful to itself but that benefits others of its species
What is flexible to interpretation is the motivation behind such an “unselfish regard”. For some, it may be the achievement of a socialist paradise, while for others it is simply following God’s orders. For yet others the façade of altruism is just a means of persuading average schmoes to sacrifice their well-being while getting nothing in return. Obviously, all three of these are very different, and have very different results.
Personally, I am not an atheist, but I do not think there is anything wrong with honest egoism (I think that is what you meant to say, as egoism = “a : a doctrine that individual self-interest is the actual motive of all conscious action b : a doctrine that individual self-interest is the valid end of all actions” whereas egotism = “a : excessive use of the first person singular personal pronoun b : the practice of talking about oneself too much 2 : an exaggerated sense of self-importance”). Just as altruism can be perverted to fit the technical meaning while totally ignoring the intention behind it, the same has been done to egoism time and time again. A true egoist understands that only those who succeed on the fruits of their own labour and thought can be honestly happy with their lives, and that the consequential aiding of others that one finds deserving is a most acceptable and good use of the profits. Private charity is not ruled out by egoism, just coerced charity by the state or a similarly force-based inititiative.
I don’t know the extent of your familiarity with Hume or Voltaire, but I have studied them both at great length, and I would be profoundly interested in seeing what parallels you draw between them and Marx, Engels et al. Personally, unless I were dividing thinkers by those that adhere to Christian orthodoxy and those that don’t, I would find almost nothing in common between them, beyond Hume’s persuasive dialectics establishing before the existence of Darwin’s “Origin” that to be an atheist was not an irrational position, but also not necessarily a correct position. In particular I think you do Voltaire a grave disservice…he may have been far from perfect but his writings are that part of the enlightenment that advanced Western thought rather than setting it back. Perhaps you meant the reprehensible Rousseau, the father of Socialism and the Societal Will?
If you are really interested in the Liberal abuse of language, I highly recommend Friedrich Hayek's brilliant chapter on what he calls "weasel words" in his book "The Fatal Conceit". I think it might also give you a very interesting viewpoint on the nature of the human delusion that reality is subject to our fancies.