Obviously Seo planned all this before the prez came after her. She might have had plans to prevent the explosion if he backed down, but honestly, probably not. She probably made a mild attempt to turn over a new leaf, but gave up some time ago and this was just validation of something she was already convinced of.
About her going all the way to mass murder, though. Normally I'm fine with detectives encountering death everywhere they go because "it's just that kind of story", but the president found it unnatural, so I guess it's not. Similarly, this could just be "the kind of story" where every other character is insane, but is it though? Should I find that unnatural? Not knowing how far I should stretch my suspension of disbelief can hurt a mystery manga.
But anyway, none of this really bothers me much because I'm way too invested in Satsuki and her turning point to mind.