Would it be bad if I back out of a job offer after accepting it?
Maybe, what do you like most about the offer?
I would make twice as much money
Sounds like a pretty good job offer, so I would not back out. Besides if you already agreed it would look bad on you to turn it down now.
I might back out after all and I don't care about how it looks on me.
It's your choice, of course, but you'll look more reliable if you take the offer you already agreed to.