See, this is the problem with the heavy condensing the anime has done. In Rin's flashback to the Itto-ryuu killing her father in episode 1, Kagehisa was supposed to give a long speech where he describes in detail the circumstances of how Rin's grandfather was chosen to succeed the Mutenichi-ryuu sword school instead of his grandfather. But the anime skipped over all that (I doubt they'll still show it now, but since there's still a chance they might, I'll refrain from saying more here), so now our main villain's motivations aren't as clear and seem more shallow or petty.
I would say being anti-samurai is indeed more accurate than simply might is right (remember, Magatsu, from episode 2, despised samurai more than anything as well). Samurai shun the use of anything but a traditional katana, whereas the Itto-ryuu are willing to use any weapon and style because all they value is victory. Samurai greatly concern themselves with things like etiquette, form, decorum, face, shame... to them, if you're defeated, you ought to commit seppuku for "honor", which Kagehisa finds absurd, as he told Rin in this episode. Instead of being so eager to die, why not live and grow stronger so you can best your foe next time? To Kagehisa, such doctrinal concerns, like whether you employ a traditional katana style or the "heretical" style of one sword in each hand like Miyamoto Musashi, are merely the symptoms of a weak and soft age of peace, that has forgotten that what truly matters is whether you triumph or perish.
The reason he let Rin live is because he's amused at how, despite her being the official successor to the Mutenichi-ryuu, she's now using throwing knives as her weapon of choice, something which would be forbidden by the traditional rules of her school. So she's now more like the Itto-ryuu, rogue warriors using all kinds of weapons and styles to kill their enemies and achieve victory, than her father and ancestors, who lived concerned with following the so-called proper tradition.