What's the glyph origin of 妖?
Why does it have a woman radical? Why is woman used in a word that means weird or supernatural?
Is there a website to look up the evolution of characters?
There is not an entry of 妖 in the 《說文解字》.
The interpretation of 𡝩 in the 《說文解字注》 mentions 俗省作妖.
𡝩 [ yāo | ㄧㄠ ]
It means that 𡝩 is used to be simplified to 妖. That is, the original form of 妖 is 𡝩.
《說文解字》 explains 𡝩 as follows.
皃 [ mào | ㄇㄠˋ ] 同「貌」。
Therefore, 𡝩 has two meanings: "skillful or clever" and "the appearance of a female smile". This should also be the original meaning of 妖.
As for the "demon" meaning of 妖, it comes from 祅.
Similarly, the original word of 祅 is 䄏.
祅、䄏 [ yāo | ㄧㄠ ]
《說文解字》 explains 䄏 as follows.
䄏 is the abnormal species on the ground.
The interpretation of this sentence in the 《說文解字注》 mentions 經傳通作妖.
《說文通訓定聲》 explains that this is a loan. (妖, 𡝩, 祅 and 䄏 are the picto-phonetic characters.)
𡝩 has two original meanings: "skillful or clever" and "the appearance of a female smile". It is borrowed by 祅, thus it also means "the demon".
I recommend 小學堂-字形演變 to look up the evolution of characters.
小學 does not mean the primary or elementary school here. It means the study of the glyph, meaning and the pronunciation of the word, including philology, phonology and exegesis.
(Input the character in the 字形 field.)
For example, 中.
I recommend http://www.chineseetymology.org. They show the ancient forms of most characters.
There the author says "From woman (Chinese ghosts are women) 女 and phonetic 夭. Meaning female ghost."
Don't know where he's read that "Chinese ghosts are women", though.
Another great source is http://www.zhongwen.com. They even have a (very helpful) printed version. There they say "Modern form shows woman 女 with 夭 phonetic. Bewitching, demon."
Now if you look for a definition of "bewitching", you'll find "cast a spell on and gain control over (someone) by magic". Considering that most characters were developed by males (consider 他/她, in the first "person" means "male"; or 安, a woman at home meaning "tranquility"; etc), it's easy to understand how women are able to "cast a spell and gain control" over men by their simply nature. Perhaps there are parallels with the way women were treated as witches in Western medieval times. Remember how lipstick were once considered bewitching:
Rather than merely discouraging lip rouge through taxation, as done to hair powder, Parliament declared that women who seduced men into matrimony through use of lip and cheek paints could have their marriages annulled as well as face witchcraft charges.