2 students(girl and boy) kill a old man and cut his body and put in a bag!!

Is shocking, is something what can make somebody go crazy; Question is WHY?
To tv, in news papers all comment, all are pro and contra;are peoples who say that was a madness moment, or was the devil hand;
So, 2 medicine students, 20 years old, a girl and a boy, kill a old man and cut his body in pieces, and put in to a bag;
Is very strange, that both was very good kids, both learn very good, both from very good and decent family. Both beatiful, both smart and with all future open to success.
First they don t want talk, but step by step they start talk and say why they make this; of course that both are in prison, both will be all life in prison, both distroy both familyes. Parents are sheoked.
So, girl need disperate money, and find an announce in newspaper, with this old man who want young girl for sex; she call him and invite in shes house, where she live with shes boyfriend, who don t was home.
There, old man promise 500 dollars, and after sexual relation, he don t want pay...and she was disperate, and kill him with a knife. Shes boyfriend come home, and because love she a lot, and maybe because God take theirs minds, in disperate situation, they want hide this crime. So, both medicine students, cut old man body and put in a bag, and want go in a forest to burn the body; But was bad luck, or God hand, when they out from home, somebody see some blood, and call police;
Now, all people ask them how was possible, but they don t know to answer;
But are people who say that old man deserve what happened; i don t think that he deserve, because is a life, is not important if he was a bad person or not; was a human;
What do you think that this was possible, when 2 young persons, make this crime..well girl is very small, 45 kg, you can imagine, what power need to kill a man 90 kg wight.
Police say that she is murderer, and shes boyfriend is complice;
What do you think about this? Exist devils here, who make peoples minds dark, or what happened with peoples?