So replaying S2 and I'm having a hard time with something

[Ranadiel]

One thing I haven't been able to really grasp with this game is how the Beast Rune comes into play, and why it's even there at all.

Arguably Luca Blight is enough of a maniac by himself to be the game's main antagonist, but how did the Beast Rune factor into all this? I'm assuming that the Beast Rune had driven him insane, like the Sun Rune can, or it amplified the aggressive tendencies that were already within him. Or, it had completely taken over his will, and Luca had become simply a vessel for the Beast Rune's own will by the time the game starts.

In fact, the more I think about it, the more I believe that's what the game's underlying message was, but I don't think that it did a good job of expressing it. True Runes influence their bearers behavior, and can dominate a bearer's personality unless their will is strong enough to resist. Luca was compelled to wage war and spill blood; Riou and Jowy were compelled to oppose each other despite both being in a position to stop the fighting at any time once Jowy becomes King of the Highlands.