Everyone dies. That's what's up with the ending.
Blonde makes the mistake of gleefully torturing a poor policeman in front of a dying but still conscious undercover cop. Orange makes the mistake of trying to save Marvin's life by shooting Blonde, who's known to be loyal to Joe. White makes the mistake of trusting Orange out of guilt, and Joe and Eddie make the mistake of coming back to the funeral home. Only Pink gets out alive… but he's caught seconds after he leaves.
When the standoff finally happens that results in four deaths, we can really see it as a series of avoidable actions that our ill-fated characters were destined to take. Even Orange's final admission to White about his true identity is a completely unnecessary action that Orange feels compelled to make despite the consequences. The film starts on a downward spiral and it just keeps going down.
What's the point, then? This is a Quentin Tarantino film, not a morality play. Don't think about it so much as inherently meaningful in some symbolic way; think of it as a natural consequence of the preceding action. Think about the lies and deceit and betrayal and violence and anger and then you'll start to see that there could've been no other ending to the film.
Speaking of the end, we'd also like to mention Harry Nilsson's "Coconut" which plays during the credits. It's another one of those upbeat '70's songs that creeps its way into the movie, providing a strange counterpart to all the crazy violent action that just went down.