Through the lens of nihilism we understand that like everything else, human beings lack inherent meaning or value. If inherent human value is the source of human equality in the abstract, then equality in the abstract itself is a worthless concept. Therefore, only our inequities, by the variant values we may develop during the course of our lives, if any, are real. A measure of man's worth is not the fact of his existence but instead what he makes of it.
I disagree. If meaningfulness is a false concept to begin with, then saying humans lack it is as relevant as saying that humans lack purple elephant feathers. In my view to latch onto the notion of meaningfulness, even if "by the variant values we may develop during the course of our lives" as you put it, is to latch onto a false concept. If meaningfulness can be tossed out the window, then what remains? When the mind and its equations are removed from the picture, we still feel emotions, our heart still speaks to us.
One can rightly pursue meaningfulness as much as one can remove their eyes from their sockets and rotate 180 degrees to look back on themselves, or that is to say, affirmation towards seeking meaningfulness is affirmation that meaningfulness is not yet acquired, yet meaningfulness pertains to that which by definition is the most fundamental and inherent of all, that which, well, 'gives things meaning'. If we are free of motives which encourage us to pursue the myth of meaningfulness, then we are free to operate according to our heart's content, without egocentric concern to elevate our self-concepts.
The ego is not based on evil, as if the flaw were a matter of morality in conflict with heart, rather it is based on the illusion that it is possible to make comparisons between people or things on the level of meaningfulness or value. And the heart, for its part, comes in many varieties, to oversimplify slightly--shiny ones, dark ones, fiery ones, etc.
To do something "because you can" is false reasoning, neglecting the reason why you chose that course of action over another. It points to the truth--when all possibilities are open to you then experience itself dictates your path organically, you just do it--yet it takes root in an intellectual reaction of fear or anger towards this choicelessness of 'ISness', resulting in a defiant validation of the ego. Incidentally, operating this way closely mimics the path of the fiery heart, and the two are often confused.
The left-hand path master neither re-makes himself every moment as if choosing among infinite possibility, nor is he shackled by the chain reactions of his past. The key is to recognize you cannot be shackled by those chain reactions because you are precisely them, and to thus work to ensure the various patterns and processes therein are not mutually-contradictory but in perfect coherence and cohesiveness. Where contradictions are found, then the heart may choose which way to go, and perhaps it is inevitable which way it will pick.
However to think it would be a rather dry and un-unique script would be to severely underestimate the breadth of permutations possible in the area of sentient consciousness. Freedom of will is a matter of who's will? Yours. Thus to use it as justification to seek out your will, this seeking is your will. If an infinitely powerful calculating machine could measure every single facet of your consciousness, as well as all of surrounding reality and all possible circumstances you could encounter, then yes it could indeed predict all your next moves. But no such machine exists, nor does any such god exist, the future is truly unknown (though in the great war the perverse lie of 'destiny' is frequently used to enslave). You have it within you to defeat any foe, any would-be deity, because you can still surprise them, and surprise yourself, this is the extent of your uniqueness and your free will, however it requires outside-of-the-box thinking, not a reactionary position against the insignificant absence of choice which indeed grants you freedom through ISness.