you could check gamefaq's story guide for the game here:
http://www.gamefaqs.com/ps3/954345-demons-souls/faqs/62697
There are many conjencture and speculation on Astraea reason for becoming a demon herself. One of the most popular one I believe is that she realized that God is actually the Old One, which bring her faith to ruin. What makes it even harsher is that sight of Valley of Defilment itself have shook her faith so much that she began to doubt in God. In turn, she become demon to help people in the Valley of Defilement. All of her escort, except Garl Vinland of course went against it, and try to kill her only to be killed by her guardian, and presume lover.
My personal speculation is that before the arrival of Saint Astraea, Valley of Defilement might be much more toxic place. The poison swamp we know in 5-2 is actually toxic swamp before her arrival. After she turned into demon, she try to purify it by concentrate all the toxic to her place, which awakens the demon baby in proccess.
Well, there is no proof, all speculation and conjecture but this is exactly what makes Demon's Souls story so enticing. It's up to you which story you want to believe, which strangers voice you will heard.