THE RULES: WIDMORE

We have seen Widmore being banished from the Island by Ben. We believe Widmore's banishment was for leaving the island and having a child (Penny) with an off-islander.

When Ben leaves the Island after Alex's death, he confronts Widmore off-island. Ben claim that Widmore "broke the rules" by killing his daughter, Alex. Widmore does not reply, except to say that he "knows" what Ben is, and that Ben had stolen the island from him.

So there are two Rule violation allegations against Widmore, which he does not expressly deny:

1. Leaving the island and having a family with a non-islander is a no-no; subject to banishment from the island.
2. Killing an islander's child (Ben's Alex) is a no-no; Ben vows an eye-for-an-eye penalty.

We can guess that killing non-island natives is okay (Ben led the Purge of Dharma). We can also assume that killing one's own parent is okay (Ben with his father) or even revered (Locke "killing" Cooper to gain favor with the Others). But to kill another clan member is wrong.