why can't women defend them self against men?

Because you guys have a steady supply of testosterone, you have larger muscle mass and a much higher ratio of strength to muscle mass than women, even if you don't exercise. Dudes are also bigger than women on the average.

But none of that has much to do with how serial killers select their victims. There's a strong psychological factor involved and it often has virtually nothing to do with how easy it will be to overpower them. Otherwise they'd be killing children or the frail elderly, right?

Men tend to be more physically stronger than women. It has to do with sexual dimophism. Men carry more muscle mass and women carry more fat. Of course there are some men women can over power depending on the strength of the female. Im a pretty petite lady but I consider myself strong among other women however 9/10 I guy could likely TKO me without effort (i could likely die). I would find to the end or try to escape from a serial killer depending on the scenario. I believe many serial killers are latent hyper-sexuals with enragement issues. Many of them rape or do BDSM like stuff to their victims

Most seem to be too lazy to learn basic self defense, as that takes time and effort to train and practice.

They prefer that a man risk his life to protect them so they don't have to.

==edit==

@"Smitten" I agree that it is honorable to help someone, but women seem to assume that it will always be the man who should help, not them. No man automatically knows how to fight; we have to take time, effort and expense to learn self defense so that women don't have to.

Because women aren't supposed to defend themselves. They must do what they're told and if they don't , they will get beaten.