Nobody has to teach you. Most women, by the time they are 9 or 10 have had unpleasant and/or scary interactions with males - a lot of them much older than they are. It's a pervasive aspect of being female in this culture and while it doesn't happen every day for most women, it happens often enough to know that until proven otherwise, men are dangerous. Bears, on the other hand, will in most cases leave you alone, where women have to fend off unwanted attention on a regular basis even if it's not explicitly fear inducing, it's still makes one feel like prey.

I have a lot of good men in my life as well, but the choice between a guy you don't know anything about and a bear is a no brainer - go with the bear. As you noted, rapists and murderers don't wear tell-tale signs. Ted Bundy was a nice-looking, friendly, charismatic guy who used that to lure his victims to their doom.

Dominance, aggression, and control of women are all part of mainstream masculine socialization. Not every guy buys into that in the same way, but enough do that it's a problem. This needs to change - for men's sakes as well. Until more men take on shifting those aspects of masculinity (which women must support) we're going to have a violent culture where a wild animal with big teeth and claws actually seems like a good alternative to a guy you don't know in a place where nobody can hear you scream.