Try to kill a domesticated dog, than go try to kill a wild wolf or coyote. A sick bastard could go around kidnapping trusting domesticated dogs and murder them quite easily. Now, try to track down coyotes and wolves in remote areas where they are hunted and shot at and fear humans and will run away, if cornered be hostile and have to be killed. One is abusing good faith and trust, the other won't trust you and will run or fight.
Bundy wasn't some sort of great super murderer, he murdered well meaning and trusting people by abusing their trust. He used charm and charisma and people's trust. Not hard to murder people who don't fight back or are shot/stabbed/bludgeoned/strangled once the murder has them under control or in his playground. Tie them up, threaten them, get them to comply, have them at your mercy, then you don't have to try hard to kill them because they are tied up and helpless. Many a coward criminal has done this, murdered people after they have complied to live. Doesnt' mean super lethality.
Gacy hired young men/older boys to work for him. Dahmer got faggots to come back to his pad for perverted things back in his controlled domain. Drugging food/drink is common. Trickery, trust, these are the most dangerous things they use.
>>601983 is right.
Prostitutes, hitchhikers, young men back in those days would drift and leave randomly and not leave any contact behind, before the days of the constant contact cell phone and internet. Victims were assumed to have went to travel the US in grand fashion to seek their fortune and adventure.
One of the biggest changes to pin in serial killers of random people is to encourage parents not to throw their kids out of the house and make them runaways, cell phones and constant contact with people means its harder and fewer for people to just dissapear with no one noticing or caring. People are also less trusting of others after all this stuff is publicized and general rise of consiousness of crime and trouble, as well as gazelleschaft which teaches and generates general distrust in everyone.