Basically, Wendell and Sarah are self-concerned thieves, having stolen the semi truck from someone else in Texas, not realizing that Morgan had fallen asleep in the back.
Their main goal after Morgan describes Alexandria (without naming it) is to make it there by any means necessary, which doesn’t necessarily mean having Morgan along for the ride.
The duo, who made the humorous claim that they were twins, don’t seem to be outright murderers, even if they did leave Morgan for dead at one point.
(Largely because he told them a bald-faced lie, which was a first for the character.)