There is a principle of 'removing difficulty' in Islam. What are its limits?

There was an article in the newspaper wherein a woman was being tried for murder. She video recorded the killing; she tied two young men (16 or 17 years old) to a tree and shot them in the head with a gun. Later, she claimed that she killed them to live; that this was her defense, "I killed them to live", since her claim was that the person who was recording the video had threatened her. He threatened that he would kill her if she didn't kill them, so in her fear she killed them for her own survival.

In Islamic law this is not acceptable. This is not a defense because the forbidden act now is going beyond yourself. You cannot take the lives of others to survive yourself. Harm cannot go beyond yourself. If you are on a raft and there are three or four of you on a raft, and there is no more food, Islam does allow you to eat human meat at that time though normally eating human flesh is forbidden. If one amongst you on the raft dies then you may eat a part of him to live. Islam allows it only under those circumstances, however, it would not be allowed for you to sit as a group and decide that so and so is the weakest amongst us, so we'll kill him and eat him. No! You can't. Do you understand where the border has to be drawn? There is a line that has to be drawn, that is, in the case of a situation of dire necessity; you may be allowed to do certain things which are normally prohibited.