Kafka suggests that societal values are distorted. We work in jobs that we hate that twist and deform us so much that we eventually become unrecognizable even to the people we love. Family tries to love us for who we are, but if we don't fit within their expectations of what we should be, they will eventually reject us; some will reject us outright. We will be loved, but only so long as we do not become a burden on the people who love us; the limits of their love may be determined not by how deeply they feel for us, but by what they think others will say about them: people will do what others will expect them to do, and therin lies their limits.
Finally, as gbeaty has pointed out, economics rule. Love, obligation, even family ties are all nothing in comparison to economics. We must survive, and in order to survive, we must make or bring in money. That which interferes with economics and survival must be eliminated, regardless of who or what that is.
Kafka's view of society is bleak. It is a society where love is not rewarded, where social laws dictate behaviour, and where, ultimately, financial considerations trump all others.
Rather than a single message, Kafka is sending a cluster of closely related messages. Start with the transformation itself. It happens in isolation—Gregor is alone in his room. He finds himself transformed, and deals with its emotional and psychological implications alone. With the isolation, a sense of confinement and limited options. This is the room, but also that Gregor is still worried about having to go to work. A miraculous (if weird) thing has happened, and he still worries about his job.
The family is the first circle of society, and they react to this change with shock, horror, and even anger; this society does not support those who are different.
They immediately form a routine, to contain and isolate Gregor. When Gregor breaks out, they attack him, society's response to the strange.
Eventually, economic forces determine what happens in this little society (the renters).