His entire purpose is to take revenge on Zero for trying to rule the world with one medium, information control, and because it was based on English.
Skullface had to repeatedly had to learn new languages, and therefore new cultures and means of communication, to serve his superiors. And he also lost his original heritage, his original culture, and his original language when his community was literally destroyed.
So when Zero is trying to rule the world with informatics based on English, something which would destroy innumerable languages and cultures and heritages like Skullface's own heritage was destroyed, Skullface understood that Zero is his true enemy.
It is a analogue to real-world information control through the real-world English-language cultural imperialism.
This is what the game is ultimately about thematically, thought control (similar to MGS2) through language (rather than information highway control, dissemination of news and the "hyper-reality" over the internet, in MGS2).
The game specifically says at a couple of points at least that people's thoughts are governed by their language, which is true. Culture is "coded" into language, as are ideas from that culture, so losing the language means losing an entire culture. Even from the very start of the game this is the argument being made:
"It is no nation we inhabit, but a language.
Make no mistake; our native tongue is our true fatherland."
-Emil Cioran, Romanian Philosopher
This is the basis for so many domestic affairs aboriginal problems in the world (Code Talker is a Navajo, and makes a point to say that his "Code" or his language is abused by the CIA for purposes of war, but his language as a whole, and therefore his culture, is dying) who lose their culture through cultural imperialism by Westernized doctrines by the majorities in their countries.
This is why the whole parasites thing eliminating specific languages is even a thing. It is the literal weaponization of cultural imperialism that Skullface suffered against in his youth and several times after when he had to constantly learn new languages based on his new employers.
Despite Skullface not being characterized very well, his character's motivations are probably the best thing about the story in this game, even more so than the twist of Venom Snake.
Kojima is basically saying through this game that information control, through creating a "hyper-reality", can be achieved by, for example, Western democracies dominating the world with the English language.
And if or when that happens in the real world, if it is not already achieved to some extent (it is), it means that the culture of the English language, and the understanding of the world through the English language becomes the standard for the whole world.
This unites the world under the English language. But as Skullface, and Kojima, are posing is that while uniting the world under one language, it imprisons everyone under one culture.
If the world is dominated by a single language, that means it is also dominated by a single culture, a single understanding of the world through the lens of that one language, and this comes from the idea of cultural imperialism.
http://www.gamefaqs.com/boards/681794-metal-gear-solid-v-the-phantom-pain/71996393
Skullface's idea of returning the world to "zero" and bringing freedom to all people was by taking their tongues, figuratively, and their language, literally, through the vocal cord parasites.
https://en.wikipedia.org/wiki/Lingua_franca
Interestingly, as Zero, Skullface, and Big Boss all strive to unite the world using somewhat similar and somewhat different means, they all equally misunderstand The Boss' intentions to keep the world the same and keep it in balance.
The Boss dies according to her true mission, which is her beliefs, in MGS3 to preserve the world as it is, not to unite it or reduce it or to create absolute freedom (anarchy).
Kiefer was only 'just ok' but he hardly had any lines anyway so it just made me think he charged by the word or something lol, can't help but feel that was another hindrance to the terrible way the story went. Even if Kojima wanted it that way, I'm just searching for excuses I guess.
