Is english your second language? How could you possibly misunderstand what I said?
"Warm for the rest of his life' is the problem. not 'forever', because a man isn't warm forever when you set him on fire. Then he's dead. It kinda spoils the joke and the understated cynicism.
Now, if you wanted to change it up, you could say "Make a man a fire and he's warm for a day, set him on fire and YOU will be warm forever." With the implication That you are going to hell for it, a rather... mild joke, but decent for all that, as it actually pokes fun at the understated cynicism implicit in the initial line, and reinforcing the christian origins of the original quote which was "Give a man a fish, and he will eat for a day, teach a man to fish, and he will eat for the rest of his life". Essentially it's a way of turning a humorous, cynical misquote back on itself.