They know that they can die even before reaching the planet, and even if they get there, it’s most likely that they will die too. And these assumptions are very difficult refute, especially when you consider that everything that ever went to the Red Planet never returned back to Earth.
However, according to NASA, it is too early for such conclusions. Since 2011, the Aerospace Agency has considered Dragon spacecraft of the private company SpaceX as a potential candidate for the possibility of its use in various Mars missions, one of which will be delivering a new Martian rover to the Red Planet in 2020. But according to NASA, it is not the only problem that can be solved with the help of this spacecraft.
NASA analysts believe that a modified version of Dragon spacecraft is safe to be sent to Mars. One of the features of this spacecraft is the possibility to transport a small rocket. This rocket can be used as a transport vehicle for sending various samples back to Earth for scientific purposes.
Note that this is just an idea and NASA has no plans to consider it until 2022 or at least unless the rover, which is planned to be sent on Mars in 2020, finds something that would be worth sending back to Earth for study.
As for the Mars One colonists, it is unlikely that a spacecraft with the ability to return people to Earth will be created at that point. However, if scientists manage to create a spacecraft that could carry to our home planet at least a pebble from Mars, the creation of a vehicle able to carry people back to Earth would be only a matter of time.