The word 'satan' is being used and heard as if it were a name. How could it be rendered in a way not distracting from its actual meaning of opposition and enmity?
In the Tanakh the concept of a "satan" exists, but it is not a personification of evil and there's no particular reason to believe there's even just one for all time. The word "satan" is a job description. The best way to render the Hebrew הַשָּׂטָן is probably literally: "the satan", lowercase 's', with definite article (the הַ). It would be misleading to render this "Satan", formatted as a proper name. An alternative is to translate the job description; this translation renders it as "the Adversary". (I don't know why they use a capital 'A'.)
Christian texts refer to a specific, evil being with super-human power. This being is named Satan. (Well, some Greek word that is rendered that way, I guess.) When translating texts using it as a proper name it would be misleading to render this as "the satan" because it's not referring to a role but a named individual.
The challenge, then, is how Christian editions of Tanakh (aka "old testament") should render הַשָּׂטָן. A scrupulous edition will render it as "the satan" or perhaps "the adversary" and rely on the reader to make the Christian connection. A more doctrinal edition will render it as "Satan" for consistency with the Christian books to reinforce the point. In the latter case a better edition would footnote the decision.