The Question: "Why is profanity deemed bad? They are just words. They can be used as nouns, verbs, adjectives, etc. So what makes these words bad? Is it just because someone said so a long time ago?"
Perhaps there are a few profane words that fit into the category of this question; but if there are, they are very few. Most words considered profanity, would fit into one of three groups:
(1) The Vulgar
(2) The Blasphemous
(3) or A Curse
God definitely has something to say about all three of these, and His thoughts are not based on the arbitrary selection of man's culture.
The Vulgar
James 1:21 Wherefore lay apart all filthiness and superfluity of naughtiness, and receive with meekness the engrafted word, which is able to save your souls.
Many profane words refer to bodily functions and sexual acts; and such topics should not be discussed in public or within mixed company and especially not in front of children who have no concept of propriety. They certainly shouldn't be something to introduce to others in anger or frivolity. Some of these words refer to the gift God has given to marriage and to use these words as by-words is to degrade God's gift.
Personally, I cannot think of any words that have been arbitrarily added to the list of profanity. Most of the words that the majority of Christian America has classified as profanity have their classification because such words refer to the crude, the base, and things which have no business being stated in public and polite conversation.
The Blasphemous
Other profane words are Bible words which are being treated lightly such as hell, damn, and especially the misuse of God's name. To take something as serious as damnation and hell and make it into a word for jokes, vulgarity, or anger is to make light of it. Does that break a commandment of God? Perhaps not, but the Bible does warn:
Matthew 12:36 But I say unto you, That every idle word that men shall speak, they shall give account thereof in the day of judgment.
I'd rather not have those words on my accountability sheet. The fact that God only warned about using His name in vain does not mean that is the only spiritual/Scriptural word that should not be used lightly. Again for a Christian, the goal should not be to see how close to sin we can get without falling in, but how far from sin can we live so that we may experience more of God's presence.
And although God's name may fit into the previously mentioned category, it also stands out above all other word; for God Himself warned of using His name vainly:
Exodus 20:7 Thou shalt not take the name of the LORD thy God in vain; for the LORD will not hold him guiltless that taketh his name in vain.
God does not "spell out" what the judgment for this blasphemy is, but one can assume it would be a weighty price.
A Curse
Then again, some profane words are "cuss" words, words used as cursing words; that is, to pronounce a curse upon whoever or whatever has angered us. To tell someone to "go to hell" or to "be damned" is to make a judgment against that person or thing as though we had the right to do so; and we don't. Only God has that right; and if He gets angry when we call someone a "fool" in anger (Matt 5:22), how much more will He be angry if we declare a person or thing worthy of damnation or hell simply because we are angry at it?
And the last thought that comes to my mind is the one some godly school teachers used to tell us when I was in school. The use of all profanity and by words is a demonstration of our lack of self-control and vocabulary. The question is not "Why can't we use them?" The question is "Why would we want to use them?" Are we so immature and uneducated that when we become tired, frustrated, or even relaxed with friends, we must let the same crude words leak from our mouth like sewage from a septic tank? Can we not control our tongue any better than that? I am convinced that profanity is so common for some people that they do not even realize when they are using it. And what does the Bible say?
James 1:26 If any man among you seem to be religious, and bridleth not his tongue, but deceiveth his own heart, this man's religion is vain.
After 12 years of formal education (not counting preschool and post high school), are these the only words we can use to express our frustrations, joys, angers, and jokes? If this is the best our education can produce, why bother with it? The educated should be articulate enough to say what needs to be said in a language and manner which does not demonstrate immaturity or foolishness--especially if we belong to Christ.
Others may disagree with what I believe, but there it is. I hope it helps.