Parenting, Social Conditioning and Child Sexual Abuse

I read a news few days back in my country that a two year old baby was raped by two men. One of them was 15 yrs old! I was shocked and hurt. It was just a small baby that these guys raped and not a grown up who could understand what was going on. I have been thinking about this for the last four to five days and I fail to understand how people could behave like this.

I just wanted to know how to protect our kids and make them aware of such animals out there. I know as parents we are supposed to start this talk some day when the kid is mature enough to understand. But after reading news like this, I am confused as to when to talk about this to a three year old or whether or not we can talk about this.

Any suggestions?