Not to an adult in their 20's they can't. They've learned the morals and systems regarding things like murder, all their lives - it's something that can be taught conceptually.

Sex? They've literally just discovered something that cannot be adequately explained (or objectively understood due to extreme hormones surrounding sex) - that adults can prey on. There is NO situation where a minor would have control in a situation with an adult, the vast majority of teenagers are incredibly susceptible to manipulation over EVERYTHING at that age.

On your second point - here's the scary thing (I've barely come to terms with it myself). Sometimes these people are just legitimately, horrible people that are in complete control - but choose to take out their anger and frustration with the world and their own failures through horrific acts of violence or negligence.

Some people are just evil, deep down. Sometimes it's their character alone, not something fostered or exacerbated by forces outside of their like mental illness.