So many apparently entitled, self-centered readers on this thread that they never stopped to re-read the initial post and ask a few questions:
1) Where does the OP live? (Sounds like not in that house. Dorm, maybe?)
2) Whose computer was it? (Sounds like a "Family" computer, which means NOT the property, in any way, of the OP, unless she paid for it)
3) How much does she rely on the family? (Money, food, college/boarding expenses, etc?) What could this person possibly hold over her as "punishment" since she's far too old for any sort of physical punishment (by about 17 years or so)
Basically, the guy is rude. He also has a right to be rude, as it is apparently his property. Too many people take a completely cavalier approach to home/family security these days. Fools put stickers on their cars announcing their children's sport, school, and name, and, by inference, their relative age and the location they live (most people with sticker'd up SUV's can't park in their own garage, if they have one). Couple this with webcams, webchats, Facebook, Instagram, etc, and a dumb parent with online foolhardy children can have any random stranger (like the ones who drive "Free Candy" vans with the windows blacked out) know everything about who and what are in your home.
No one on this thread knows enough to answer the OP's question of whether she has a right to be angry. She hasn't given us anything but her story. It certainly sounds like she's mostly on the right track, but it also sounds like she's still heavily reliant on her mother's household.
The stepfather may have acted like an asshole, but that doesn't mean that he didn't have the right (his property) to do so and that he wasn't right to do so. I am almost as far as you can get from digitally/electronically illiterate, and if I found my daughter speaking to people regularly via webcam, and I wasn't 100% sure who those people were (i.e. I had met them personally), then I would bring down the hammer on that immediately and with enough force to make sure it didn't happen again.