Thanks for translating this and the second part. I've enjoyed this author's take on the darker background for those characters. Seija always struck me this evil, even in mild Touhou canon.
Seija seems really out-of-character here. While she's definitely belligerent and mischievous to the point of malice, she was never really a straight-up murderer. And targeting Sekibanki like this doesn't fit her MO, either. She's usually all about flipping the balance of power (or imbalance of power, as she sees it) as opposed to sending everything spiraling into chaos. "Youkai should act like youkai" is more in line with either Shikieki's or Yukari's line of thinking.
Still, I guess it's an interesting plot-twist that Sekibanki's offhanded wish helped create Gensokyo.
It was a different time and a different age. Most youkai probably don't even consider killing to be a heavy action and treat death nonchalantly. If Suika was Shuten-Douji then she has a long list of terrible deeds herself before Gensokyo.