Cancel Out?

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>>In Winter's Heart, Rand and Nynaeve al'Meara used the One Power to set the evil of Shadar Logoth against the evil of the Dark One's taint on saidin. The two evils wiped each other out, and Shadar Logoth was completely destroyed, leaving a massive crater.

My impression was that Rand chose Shadar Logoth as the location to cleanse the taint not because it's properties would aid him in any way, but so that if something did happen like the large crater, at least the crater would do some good. I have never heard that the two evils canceled themselfs out, thought that is interesting seeing as Rand's two tainted wounds throb in opposition.