If a round trip is cheaper than a one-way flight?

No airlines wouldn't go after you if you don't use the return segment of the ticket. I had done that before for the same reason. In fact if you don't show up or better cancel it ahead of time, they could resell your seat for more money.

You have to read the fine print, but I've done this before and was never penalized.

The only thing you have to watch for is that many airlines will cancel the rest of your flights if you miss one flight. So, for example, if a flight from New York to Rome to London and back is cheaper than just flying New York to Rome and back, you could theoretically just get off in Rome, ignore the Rome-London and London-Rome flight, and just fly back from Rome. However, they could cancel the rest of your flights, including your Rome-New York flight.

They would just cancel your return portion.

It would be best to contact the airline to cancel it (instead of not showing up) so they can resell your seat

If you split the round trip in half (ie to get the theoretical cost of each way), it will end up being cheaper.

However, the one-way cost is definitely not as expensive as the return flight.

one way might be $100, round trip $150. Is it cheaper? Split the round trip in half, yes. Otherwise, no