In general, if you find a way to pay less, the airline won't let you. In particular, if a return cheaper than a single you can bet then the airline will prevent you from using a return as a single if it possibly can.
This depends on the ticket rules. Usually, the rules depend on the airline. Usually, “traditional” airlines sell there-and-back tickets, where a return costs a lot less than two singles (sometimes less than one single), and you must use all the segments in sequence: if you don't take the first flight, then the airline will cancel the ticket (examples: British Airways, Singapore Airlines). Usually, low-cost airlines sell independent tickets, where a return is just two singles (and priced as such), and in that case you can use the return if you missed the first flight (examples: Easyjet, Ryanair, Wizzair). Note usually — there are plenty of exceptions, you need to check on a case-by-case basis.
Airlines set prices based on yield management, not on cost. If there's an apparent way to bypass yield management, then usually the airline is wise to it and has made a rule to forbid it. For example, if an airline has noticed that only a few rich travelers do complex travel involving one-way flights, then they'll make single tickets very expensive. If they do that, you can bet that they'll try forbid anything that allows people to book a return ticket and only use half of it. Technically, using the first part of the ticket but not the second one may be a breach of contract — but the most the airline can do if you miss the second part is to cancel the ticket which you're no longer using anyway. But if you try to use the second part without the first, the airline will cancel the ticket before you use the second part. Airlines also sometimes decide that people from A are richer than people from B and so sell A–B–A for more than B–A–B. Again they arrange to prevent people from A from using the cheaper fare. (Sometimes if you fly the route multiple times you can get one A–B–A ticket for the first flight and for the last return, and B–A–B tickets for intermediate flights, but this is often difficult because you need to know the dates long in advance.) The same kind of reasoning explains why a ticket with a changeover A–B–C is sometimes cheaper than A—B (people from C are poorer, or there's more competition for A–C than for A–B) — and again the airline won't let you take the return flights if you haven't gone all the way to C.
Low-cost airlines often do their yield management differently, on a strictly per-flight basis. If the airline prices A–B–A as A–B plus B–A then it has no incentive to tie the second flight to the first.
In summary, if there's a “nonstandard” way to get a cheaper price, you can bet the airline has thought of it and has arranged to forbid it.