Think about the past few generations in the family: Ishmael misbehaves so he gets kicked out of Judaism; Esau misbehaves so he gets kicked out; and now Joseph is telling dad all sorts of stories about how they're misbehaving. Rabbi Ovadiah Sforno points out at this point, the brothers believe Joseph is going to get them cursed, kicked out of Judaism, have dad pray for them to die, or the like -- he is actively threatening their bodies, souls, or both.
Judah's response is that even when someone is coming after you with a knife, you don't use lethal self-defense unless it's necessary. (Or per the Talmud's expression, "could you save the would-be victim by just taking out one limb of the attacker?") Once there are traders nearby, he sees a perfectly good less-than-lethal solution: sell Joseph off to some faraway land, at which point you've neutralized the threat.
Sforno's approach is that this was all handled the same way that a civilian would in assessing an active threat to his/her life; that way, we don't get into complicated questions about the official legal punishment for something done in the past.