It depends on what you mean by "a bad word." A word is just a sound. Sounds are neither good nor bad, they just are. The meaning and connotation of some words causes them to have a negative stigma. "Hell" has a negative stigma. It's not the sound's fault. We know that "Helicopter," which contains the entire "hell" sound isn't bad. So what makes a word bad is the meaning. Cultures decide this. Ours has decided that Hell is a bad word, though it's mild compared to some other "bad words."
I don't think hell is a bad word. It just expresses, the extreme negative thought of something. As a matter of fact I use it all the time, to refer to something that is really bad or unimaginable, because that's what hell is like.