" Why isn't Christianity considered evil?"

because the vast majority of Christians know fuck all about their own religion, their holy book or their doctrine and dogmas.
The and the vast majority of none Christians today associate it only with Christmas, boring Sundays and school assemblies.

When the church was regularly burning, hanging and drowning people, administering local law, beating adulteress women in public and taxing the local populous on behalf of gawd, raping women and assaulting children, their opinion of these wretched clerical leaches, liches and letches was more fittingly derogatory

I understand the arguments espoused in the article and I agree with them. However I judge the degree of "evil" (when considered as the set of ultimately destructive choices or the framework by which those choices are valid) of a belief system by the actions it has contributed to or inspired. As such I think xtianity is both abhorrent ( the Inquisition) and wonderful ( Mother Theresa). There are very few belief systems which I would consider entirely evil - perhaps Nazism or Fascism in general. I also think that when measured using this paradigm capitalism is mostly evil. And xtianity is probably closer to being evil than good. But it's mostly just false and ignorant of empirical facts.

Some people do not recognize evil even though it is staring them in the face

I consider it evil, the way it's practiced by many (most)

It is. By me.