Let’s give people a God, let’s even make him the God of the bible…..
You ask the question, "Even if God exists why worship him?" However, you quickly turn it into "the God of the bible."
When it comes to that question, we have to be more specific. There are many gods listed in the Bible and we find out about more as time goes on. Is it El, YHWY, the Lord, or even Jesus (if you grant him divinity).
I have come across some interesting information over the past couple of years where it shows the Israelites had several gods during the course of their history. There are even those who suggest that in the Gospels Jesus rebukes the religious leaders for worshipping YHWY when their true god is this "Lord".
I get your argument about why any would ever want to willingly worship the god of the Old Testament. Keep in mind, part of that is because morality is dynamic and not static. Religious belief is part of culture. Culture is a learned experience. Over the centuries it begins to look like "that's the way it has always been." This is not true.
So, I think another way to look at it is "How could anyone in our current culture and in this modern day and age choose to worship the brutal god of the old testament?" I think the reason is simpler than we think - we were raised to believe it.
True, some people choose to accept this blood-thirsty tyrant as their god later in life. I have no idea why other than the shared experience. People find something to belong to and want to share in the emotional experience the others in the group have.
I get wanting to follow the tenets of Jesus. However, Christianity tends to say "You have to accept the YHWY if you want the Jesus." That is hard to come to terms with.
I also see Vidqun's point. Who am I with my first world, suburbanite morals to tell God what is right and wrong?