It is the most fitting theme for the second half of the series where Light's character development goes through it's logical conclusion ending in spectacular fashion.
The song constantly refers to human (Ningen) in the third person as fearful and weak with the singer as a smug and mocking observer who progressively gets angrier and angrier until he devolves into just a gutural growl which in the full song ends with them coughing due to lack of air.
It goes from "Ningen sanka. Ai nige ningen fuan ka" which is mockery of the human and their nervousness into just Ningen Fucker, Ningen Sucker, it perfectly shows what starts happening on the second half.
Considering the relatively short length of the series I'd say they start using it ridiculously early. Especially considering it still doesn't quite fit perfectly even at episode 29.
I'll see? I'm not gonna look back and see that this theme fits the BUILD UP. Even if it does eventually fit a couple episodes toward the end using it almost 10 episodes before it even remotely could fit was a mistake.
I'd argue it still doesn't fit even after watching episode 29. It's getting there. Also I only appreciate it as an immediate? Wtf is that? It's still good just has an ill fitting theme for roughly 1/4 of the series. At any rate how is only appreciating something for it's immediate merit bad? Just becuase something is good early on doesn't mean it always will be. Look at TWD GOT or weeds.
TWD was never good, and Death Note is a more compressed show with a predefined ending and length. Those are about the whole not the episode by episode.