Some people use foul language often. Others rarely. And some others never. The use of bad words is a reflection of the person. But sometimes it's a matter of habit. When bad words are spoken we are not able to gauge whether it is habit or personality. If it's a habit, then we can let it pass. But if it's a reflection of an aggressive personality, then it is scary. We can never know which one it is, hence bad words are scary.
The use of loose language reflects our looseness. It shows our carelessness with words. Moreover, it reflects a lack of human sensitivity. You can't expect people using foul language to be sensitive. It does not go with the persona. Bad words reflect our animal instinct. We have gone backwards in our spiritual evolution. Instead of divinity, bad words pulls us towards bestiality.
Can bad words be spoken with love ? Can bad words have care, and compassion in them ? Can bad words be soft, and intimate ? No, they can't. It removes man from his own soulfulness. He can't connect with another soul. It increases distance between humanity. When bad words are spoken, we feel disturbed, we feel nervous. Hence, bad words are scary.