Thinking about it just now, he probably fully believed he could turn Vader at that point. The very next thing Luke says is a plea for his father to help him. Just from a worse position than he thought.
The minions can't be martyrs. They don't have the eyes of the public on them. If there are enough of them (eg Death Star population), there is no way to stop the Death Star at that time while not killing them. None of these are strictly, necessarily true for the leader. It sucks, but that's how it is sometimes.
Not all the times. But sometimes.