What is the Music called which is played when antagonists are plotting something?
You may be looking for Kokoro no Hadaka. It's typically referred to the Raijuta theme. It's by Ueda Yuji. 

https://www.youtube.com/watch?v=lHl_sYLLktw

Here's a list of all of RuroKen's OSTs https://en.wikipedia.org/wiki/List_of_Rurouni_Kenshin_soundtracks

This one is the fourth track in the Rurouni Kenshin Original Songs I soundtrack.