I do like Annie's analysis of the problem and comparison to the animal world. However, even though I've not studied animal behaviour in any serious manner, I'd say that they don't have the capacity for evil. Today, we don't think "Evil" exists. But Evil does exist, it's the opposite of what we commonly call good. When we talk about love, Evil is easily the opposite of what we traits we admire, from the bible:

1 Corinthians 13 4-6 states:
4."Love is patient, love is kind. It does not envy, it does not boast, it is not proud. 5. It is not rude, it is not self-seeking, it is not easily angered, it keeps no record of wrongs. 6.Love does not delight in evil but rejoices with the truth. 7.It always protects, always trusts, always hopes, always perseveres"

My wife has dabbled in "according to her" eastern meditation. She would often sit there in bed with a blanket over her head and say she was meditating. Some forms of meditation are known for chanting the names of deities, which "may" be the names of demons. If you think this is not possible, widen your gaze. There are still many perhaps previously known, unknowns out there!

I personally have a hard time with Evolution. I don't see artifacts that prove Evolution and I'm no blind creationist either. So for Annie to state that perhaps this condition is a predatory behaviour from our past species gives me caution. I'm more likely to believe it's from a combination of personality, experience and environment at a young age. So the person has personality 'A', experiences specific events and lives in a specific type of environment that cause this behaviour. And perhaps the environment doesn't affect the other siblings in the same way because of their personality, age and the subtle difference in the environment during formative years, however does affect them, just differently.

More Posts