Is it normal when someone writes something nasty on youtube comments and?

So this seems to happen more and more and it's really confusing to me.

The last time it happened, it was on a video of a mentally disabled man talking about how he's bullied at school and some jerk made a comment, "what a stupid ugly child, why don't people abort these gross creatures?"

My reply: "That is such a nasty thing to say, why would you say such a thing?"

His response: "What the f*ck is a matter with you? You know I'm f*cking allowed to give my opinion so why does it f*cking matter to you? I don't give a sh*t what you think. My opinion is right because I have a brain and I know what's bad for the world. People like you end up alone in life because they don't see how ugly the world is. You're going to be the person who marries a fat ugly stupid cheater with a unibrown who doesn't make more than mcdonalds salary a year and then you'll realize why I'm in the right and you know nothing. I can say this as a fact to. I know you'll end up with someone just like that."

And then I replied with: "I don't understand where all this hostility comes from. I really think you need to chill"

Then his final response was, "OMFG what is with you trying to start a fight? you're a f*cking a**hole. I have an opinion f*cktard. I don't get why you defend a retard that doesn't know left from right. These people need to be aborted and you know it. Just go leave me the f*ck alone and stop starting sh*t for no reason. I f*cking h*te rude people. You think your opinion is the only thing that matters. I gave my opinion, I'm allowed my f*cking opinion and I'm also allowed to be right about it. Go find your fat ugly morbid obese stupid unibrown mcdonalds hamburger flipping husband you c*nt."

....Does this make any sense? I never understand these people. They give their opinion and I give mine. I do not fight them nor do I say anything rude and they go off on a tangent and don't shut up and call me rude when I'm not saying anything rude at all.

Is this actually normal behavior? Does anyone else encounter this? I can't understand it. Another funny thing is, I'm engaged to a guy exactly opposite of that.