Update on both living with the victim and being female and the small probability is bigger but... still small, since the still relatively low probability of a roommate murdering is penalized substantially by being female
Low compared to what? For someone murdered at home in the dead of night, the dominant probabilities are that either the murderer was invited in or lived there. Roommates merit investigation. If the evidence clears spouses/lovers and close family, then the probability of it being a roommate goes up considerably. Being female is not going to lower the probability enough to eschew a thorough investigation.
What saves Amanda Knox in this case isn't being female, but rather evidence that someone else committed the crime, as well as the lack of physical evidence of her involvement or any paper trail pointing to a conspiracy.
For someone murdered at home in the dead of night, the dominant probabilities are that either the murderer was invited in or lived there. Roommates merit investigation. If the evidence clears spouses/lovers and close family, then the probability of it being a roommate goes up considerably. Being female is not going to lower the probability enough to eschew a thorough investigation.
You're not disagreeing, but you're failing to consider the numbers here. If, say, a quarter of people are murdered by their roommates, and males are 10x more likely to be kill...
Here are the New York Times, CNN, and NBC. Here is Wikipedia for background.
The case has made several appearances on LessWrong; examples include: