Plan A - The Brothers':
First Degree Murder
Recall that as soon as Yosef arrives at Dotan, where his brothers were, the brothers conspire to kill him (see 37:18-20). However, their plan of how to kill him is revised several times. Let's begin with Plan A:
"They (the brothers) saw him from afar, and before he came close ... they conspired to kill him. And they said to one another, 'Behold the "dreamer" is coming. Now, let's kill him and throw his body into one of the pits...' " (see 37:18-20)Note that the brothers first plan to kill Yosef on the spot and 'bury him' in a pit, and thus 'cover-up' any future evidence against them.
Although Reuven opposes Yosef's murder, he realizes that the brothers will not accept his opinion. Therefore, instead of arguing with his brothers, he devises a shrewd plan that will first postpone Yosef's execution and later enable him to secretly bring Yosef back home.
[See "Further Iyun" for an explanation of why specifically Reuven wants to save Yosef.]