If you tell them you only intend to fly one way, an airline may try to get you to pay the one-way fare - which, as you noticed, can be higher than the round trip. So, first check the specific fare rules and what the one-way fare would be. Let that guide you.

If you just don't show up for the return flight an airline is unlikely to come after you for the difference, since there are any number of legitimate reasons someone might miss a flight and then make other arrangements to get home. If you make a habit of it, they might get interested in why you're so unlucky with traffic.