The curse and its rules (spoilers)

I haven't seen this subject discussed here, so...

When Martin explains the "curse" to Steven the fundamental rule is "just like you killed a member of my family, now you gotta kill a member of your family". During the whole movie I didn't feel that the father was not one of the options, in fact I spent the whole movie thinking he was just going to kill himself. Martin never implies that Steven is not a member of the family and since he was responsible for the death of Martin's father it would be the "right" thing to do to sacrifice himself and save his children and his wife (all three innocent of Martin's father fate).

Even if Martin also explains to Steven "don't worry, you won't get sick. You've gotta stay calm, that's all" and putting aside all that Greek mythology, it's still the rightful thing to do. In fact, the quick explanation by Martin and especially the "you've gotta stay calm, that's all" seemed more like a riddle. "Stay calm, think logically, you have to kill one member if your family... Just realize that it can be you, and do the right thing".

The ending of the movie left me guessing if Steven didn't understood the rules, or if he did but he was so selfish that even knowing that suicide was an option, he'd rather kill another member of the family instead. Steven's character seemed far more dark than what is implied.