Is Kurtz playing by the rules of the jungle, which differ from those of a civilized society?
I'd say no. Kurtz is playing by his OWN rules. Marlow mentions that the cannibals with whom he is traveling have "restraint." They live by the laws of the jungle.
Kurtz, on the other hand, shows absolutely no restraint. He is no longer within the reach of "civilized society," and without the laws of his former society to guide him or any checks to keep in accordance with those laws, he abandons the restraint those laws placed upon him. Nevertheless, he does NOT live by rules of the jungle either. He does appear to take on the culture of he natives, but this transformation is an illusion. Yes, he becomes wild. Yes, he surrounds his hut with shrunken heads, and yes, he even participates in their cannibalistic rituals—but not in the same way that the natives do. He only participates insofar as these activities give him control and power. He does not participate in the natives religious rituals as a pilgrim or a worshiper, but rather sets himself up as a GOD. And in doing so, he abandons not only the laws of his own "civilized society," but the laws of the jungle as well.
With neither set of laws to govern him, Kurtz is left to his own devices. And he proves the point of the book—that all men, no matter how "civilized" they may be, have the potential for utter darkness at their core. He proves that without societal checks, which would include the "rules of the jungle," even the most civilized and enlightened man can and will abandon all restraint.