Mom sentenced for killing baby in microwave

I was horrified when I read this. How can a mother do something like this ? There must be some level of insanity that must be running through this lady, else how could a person actually put their own flesh and blood, their own baby into a microwave and kill the baby that way ? This lady has now been convicted of this crime, and sentenced to life in prison:

DAYTON, Ohio (AP) -- A woman has been sentenced to life in prison without a chance of parole for killing her baby daughter in a microwave oven. Arnold was convicted of aggravated murder in the 2005 death of month-old daughter Paris Talley. She was spared the death penalty because jurors could not reach a consensus on the punishment.
Arnold was accused of killing month-old Paris Talley in 2005. A judge declared a mistrial in February, and the retrial began August 18. The jury found Arnold guilty Friday of aggravated murder.

It is shocking that anybody could put a defenceless 1 month old infant into a microwave, and then proceed to actually go ahead with the task of putting the microwave on and killing the baby.