To answer your question directly, « hell » is considered a profanity because it is (or was) frequently used as a curse and it is (or was) considered blasphemous.

Is hell a bad word in the Bible?

The Bible continually warns of a place called hell. There are over 162 references in the New Testament alone which warns of hell. And over 70 of these references were uttered by the Lord Jesus Christ! The man in Luke 16:24 cries: ". . .

Is Heck or hell a bad word?

Heck is a bad word. “Heck” is a modified word of “Hell”. This is why you won't hear their modifications in a formal speech or gathering because they remain what they are – profanity words. It comes from Northern English as a kind of profanely euphemistic alliteration of "hell".