Also why people always assume the killer is male

Of course it has nothing to do with the fact that statistically most women kill in non-violent manners such as poison, and the ones that are violent tend to be apprehended quickly with little to no question of their guilt. Thus making the story less appealing to the masses than a long drawn out serial killer spree in which bodies are left in gruesome ways to be found. Granted learning how to get blood off things is a useful skill and you never know when you'll need to help a friend hide the evidence of the gruesome murder they just committed.

Preach.