There has been too much foul language here of late.

[Philippus Flavius Homovallumus]

Why is that offensive? Are they God? Do they have the power to curse someone to hell (assuming it exists)? Swear words are just words. Nothing more, nothing less.

Basically, this:

In this specific case, it's not whether that person actually goes to hell, but that fact that someone would wish it upon them (even if it isn't done with serious intent).
Swear words don't physically hurt people, but they can hurt the sentiments of some people, who might not be thick skinned enough to withstand verbal blows. It's rare these days, specially online, but not non existent.

Spanky, you don't understand curses.

The Latin word is maledicto - an evil speaking.

"Go to Hell" is the same as "I wish you were dead."