I think that Kafka chose to turn Gregor into a nasty cockroach because if he chose a cute animal it would not make the family's attempt to care for Gregor as interesting as it was. If Gregor was a cute butterfly i don't think his father would bombard him with rotting apples. Kafka knew that if he made Gregor an adorable, loving creature, then he would not be able to show how frustrating and scary it is for his family to care for him. If he did not make Gregor a nasty insect then that would not have tested the family's ability to care for Gregor. which would make a boring story. I also think the cockroach symbolizes how Gregor and possibly the author him self, thought about themselves. I think that he was trying to say that some people don't deserve to be human unless you live life. Instead of just waking up and going to work, and then go home and sleep, and then repeat that cycle for the rest of your life.

I think he chose the roach because no one cares about cockroaches. His sister was the only one that showed that she still cared about Gregor, but that feeling quickly faded. Nobody wanted anything to do with him. which is why he was treated like trash and was being imprisoned in a nasty room which was used to store all the useless crap that was around the house.