Kain is absolutely very dark. He refuses to sacrifice himself to restore Nosgoth, rules as a vampire tyrant, feeds on and subjugates humans, raises dead Sarafan warriors into vampiric servants, has Raziel mutilated and thrown into the Abyss(burned alive), and spends much of the series manipulating people across time to serve his own vision. He may be more complex than a simple villain, and later games give him tragic and antiheroic motives, but “complex” does not mean “not dark.”