This is -- at best -- laughably inconsistent.
One of the primary reasons for carrying out beheadings in public or for perpetrators to video a violent act is to send the clear message to its audience: this is what our "law" or "power", or violence can do to you.
So, the re-showing of such footage on Facebook is actively collusive with those actions. It disseminates the fear and intimidation intended in the act.
Asserting that Facebook users can respond to footage of a woman's brutal murder by decrying it, sidesteps that issue. Users could just as well decry violence against women without seeing this act. But fewer of them may do so.
If Facebook really was interested in public debate, it would have established a real and carefully constructed, open forum in which this decision could be debated -- as well as other issues about its policies, operation and inconsistent stances.
What Facebook is interested in, is generating more traffic through its platform and it is doing so from within a pretty inconsistent, narrowly male and conservative image of the world and of what should be discussed within it.
How different is that from many traditional media organizations of the 19th and 20th centuries?