Was The Cook (Mickey Rourke) essentially assassinated by The Man (a foppish Eric Roberts) to prevent him from possibly testifying against him or was the Meth Lab trailer explosion during the film's denouement purely accidental?
The ending can be interpreted in one of three ways; you may decide for yourself:
One: Perhaps The Cook was a liability to The Man as he was one of his (likely many) methamphetamine cooks with intimate knowledge of his operation. If The Cook were brought to court and charged with the slew of felonies he had committed (including, but not limited to, reckless endangerment of the denizens of the motel; attempted manslaughter, again, for blowing up the motel: 5 years in prison for each charge in California; operating a Meth Lab: 7 years in prison in California) he would very likely "flip" (turn state's evidence to save his own skin [being a "death dealing" methamphetamine cook and criminal, he wasn't exactly a "stand up" guy]) and testify against The Man in exchange for immunity from prosecution, or at the very least reduced charges. The Man obviously had quite the methamphetamine production operation going, which is evident by the fact that he had the required ingredients and equipment needed to manufacture methamphetamine on hand, as well as the sizable roll of "walking around money" that he proffered The Cook.
He posted The Cook's bail (likely through a "friendly" bail bondsman; first to avoid any connection between himself and a "suspected" methamphetamine chef, and second, it was likely a bondsman who wouldn't search too hard when The Cook failed to show for his court appearance--The Man in all likelihood having already paid the full amount that the bondsman would have been on the hook for to the court) and then "took care of" the problem by incinerating The Cook so that he could not be found and brought to court and then possibly be made to testify against The Man. By giving The Cook the money and acting friendly toward him (and they likely were friends, but business is business and he, like most criminals--and people in general--valued his own freedom and life above that of others), he didn't arouse his suspicions.
His line when The Cook was leaving The Man's "den of iniquity" is somewhat telling, "Now there goes a real man." This could be interpreted to mean that (and this may be a bit of a stretch) he "was" (past tense) "a real man." Otherwise why not say, "Now that's a real man." As he leaves, The Cook suspects nothing is amiss and is subsequently blown up in his new lab; perhaps one of the (very) inflammable liquid containers was rigged to explode upon being opened, but this is pure speculation... but it is fun to speculate.
Two: Pure accident. The Cook had already turned one of his labs (the motel) to cinders (and likely others not shown in the film, it is an inherent risk of the enterprise), so it isn't too much of a stretch to believe that he could blow up yet another one, and this time he was unable to escape in time. During his phone call to The Man while in county jail, The Man even says something to the effect of "Uh, turned another one into an ashtray". This can be taken to mean that The Man, having worked with The Cook for so long, trusts him to keep quiet and knows that blown up Meth Labs are just another cost of doing business.
Three: It could have been The Cook's way of committing suicide. The story that he tells to a sleeping Ross while en route to the camper/Meth Lab about his mother drowning puppies she couldn't care for and how she wished that she could have done that to him as well, shows a vulnerable, sad side of The Cook (one that he likely would never show in public) and his frame of mind at the time. His girl Nikki had just left him to go back to Vegas (and though he says she'll come back, he doesn't really seem to believe it), he had just been arrested for blowing up his Meth Lab in a motel and was facing some potentially lengthy jail time; he was just an old, used up crook... maybe he had just had enough of that life and this was his way out; to go on his own terms. It wouldn't look like a suicide and so he wouldn't appear weak for having taken "the coward's way out", it would look like an accident, par for the course for a methamphetamine cook.
Personally I hold with the first possibility that I posited. I believe that The Man, being savvy enough to run a large illicit drug manufacturing operation without arousing the suspicions of law enforcement, wouldn't take the chance that one of his employees (whether a friend or not) could testify against him. He may have done it with a heavy heart, as he and The Cook were obviously so intimately acquainted and may have worked together for quite some time and they both shared a bizarre affinity for the utterly asinine Insane Clown Posse's Juggalo Championsh*t Wrestling....
The more I think about it, though, it could've been his suicide. Regardless, I do not believe that it was an accident.