Robot Style posted:

According to interviews, they wanted to kill a main character to show that this was a story with real stakes where anyone could die. They originally wanted to take out Luke, but Lucas nixed it. The droids were also off-limits, since Lucas intended them to be the ones who told the Star Wars story to the Whills 100 years after the movies. Apparently Dark Horse editor Randy Stradley came up with the idea of "killing the family dog" as a way to keep the more important human characters alive.