Unfortunately there are hardcore Catholics who act just like this toward Protestants too. Though I concede many are converts from Protestantism. Really makes you wonder what those Evangelicals put in the water or soda pop or coffee and donuts, whatever. It ain’t Christianity. I realize we all sin but we need to try and whiten our black (sinful) hearts so to speak. And those folks get black. I sometimes worry we should be evangelizing them more if we really loved them.
I deal with this too. Online I would not waste much time. I think it is very entertaining and sometimes actually productive to exchange ideas, comments with others on a forum. But if you are hit by a troll, someone who just has it in for the Catholics or the Protestants or whoever, sign off. You are wasting your time. Doesn’t take very long to figure out if the person is really interested in exchanging information or attacking.
With sincere anti-Catholicism, Protestants who really believe all of the slander, I just explain the Church’s position. This happened to me with a Protestant friend recently. She has not touched the subject since. I am not saying you’ll win every time. But I would say stick to your guns, it’s ok if you lose your temper but try not to. Just remember you represent the Church. You are more effective acting like a Catholic (Christian) than talking about being one. I think I got across - ironically - how much better Catholics deal with ugly anti-Catholicism from Protestants than vice versa. We stay respectful, civil. She got it. Could be temporary though.