I play on the Xbox 360 and am going on a run around Skyrim trying to kill nearly everyone I meet. But there are a lot of NPCs I can't kill because they are apparently "essential" and give quests. I have failed a main quest by killing Alvor in Riverwood, but people who are involved in the main questline (e.g. Jarl Balgruuf) still won't die. Is there any way to kill them?