Jesus was a radical, in his day. He upset the status quo enough that the guys in charge murdered him to silence his teaching. What did he say that was so radical? Stuff like "love your neighbor." These quotes are from the Old Testament of the Bible, whose purpose is to provide context for the stories about Jesus. So the reader would understand exactly how radical Jesus's teaching really was, by showing what society at the time considered virtuous.
If people want to kill homosexuals because the Bible says so, they should also be demanding that we stone anyone who disrespects their parents. If you're going to follow arbitrary rules "because God said so" you have to follow ALL the rules.