I'm trying to come up with a way to express the opposite of "mutually beneficial" in a way that is just as efficient. The idea I'm trying to concisely express is that a deal, agreement, contract etc. benefits only one party instead of both.
One might say the deal was asymmetric or one-sided.
Asymmetrical — M-W
having two sides or halves that are not the same
In either case it seems clear that one party is getting the better end of things.
Unconscionable is a legal term to describe just such a contract or deal.
In contract law an unconscionable contract is one that is unjust or extremely one-sided in favor of the person who has the superior bargaining power. An unconscionable contract is one that no person who is mentally competent would enter into and that no fair and honest person would accept. Courts find that unconscionable contracts usually result from the exploitation of consumers who are often poorly educated, impoverished, and unable to find the best price available in the competitive marketplace.
Contractual provisions that indicate gross one-sidedness in favor of the seller include provisions that limit damages against the seller, limit the rights of the purchaser to seek court relief against the seller, or disclaim a Warranty. State and federal Consumer Protection and Consumer Credit laws were enacted to prevent many of these unconscionable contract provisions from being included in sales contracts.
I believe the word you are looking for is 'zero-sum'. The notion of zero-sum has its roots in game theory, but the concept of a "winner-take-all" outcome to an interaction between entities can be found all over nature. For example, when an ecosystem has reached its carrying capacity, the survival of a group/individual comes at the perdition of another group/individual.
If you wish to explore more abstract, universal manifestations of zero-sum circumstance, see also Conservation Law.