Seija is everything about contradiction. Her nature to turn thing upside down is basically translated into "the weak defeats the strong" ideal. But the kicker here is that when the weak defeats the strong, that means the 'weak' is not really weak but strong. Thus she takes upon herself that she will always fight the 'strong', even when it's previously the weak.
When Seija wins against Gensokyo's big-names in ISC, her action of using cheat items caused Soranaki to appear. Since she wins, that means she's strong, and the Soranaki is also strong too because it appeared because of her. So Seija goes on to destroy the Soranaki because 1) It's obviously the strong because its existence causes Gensokyo in turmoil and 2) It reminds her of the sun who 'won' the staring contest against her, which means it stronger than her.
