You must always use the outbound ticket, or they will cancel the remainder of the ticket.
You are suggesting, you use the outbound and then no-show the return. You CAN in theory do this, however the airlines are aware of this trick, and generally have something in the small print that says they can charge you the extra a one way would have cost if you do that. That said, i am unaware of many cases where that has actually been done. If you were pulled up for it, its a case of pleading, "oh dear i missed my flight home cos my car broke down".
So, you can do it, but there is a small risk involved. So its up to you.