I think it really varies on a case to case basis.

Sometimes they're simply roleplaying being a dick, other time's they may just feel inferior in RL and feel the need to express dominance in game, sometimes they take pleasure from people's pain, other time's it's just how the game plays (will touch on this more later) etc.

I'll admit I'm not 100% innocent in this though to start. Sometimes when I play Planetside 2 when waiting for my friends to get on and stuff I'll sit in the warp gate in my sunderer. And when I see someone spawn a ship I ram my sunderer into them trying to flip them over. I really just do it for the reactions, both from how the people flying the ship freak out at times and how the friends I'm with over Skype go "Anthony! What the hell!?" while laughing.

Now with Day Z, Rust and Dark Souls I don't do shit like that. 

Dark Soul's is a pain enough single player to fight the NPCs, I prefer to avoid shadow fights where I can. But it's right down written into the lore and plotline as to why people are unfair to new guys.

For example let's look at Lautec and Solaire.

Lautrec: https://www.youtube.com/watch?v=o95uUu7I9Xs&list=UUe0DNp0mKMqrYVaTundyr9w
Solaire: https://www.youtube.com/watch?v=skV-q5KjrUA&list=UUe0DNp0mKMqrYVaTundyr9w

Notice how Lautrec largely reflects the kind of player who would grief and do anything for personal gain? While Solaire largely reflects the social and friendly player who wants to help people and play alongside them? Both reflect dark souls, both reflect paths the player can take with your own playthrough. I won't argue for a second that it didn't have some kind of negative effect on new players, but it was a purposeful design choice in dark souls to better drive home the world you are playing in.

Day Z and Rust is also part of the design. You are in a post-apocalyptic world with everyone fighting to survive. People preying on one another for survival is natural, it's what almost any game or show with that theme (Fallout, Walking Dead etc.) highlight. But there's still choice, do you go the easy route of raiding others for more supplies, or take the hard route of trusting people and working together to make a better society?

Personally in Rust I tried the later, and we soon as a successful village of 6 people going on. Sadly we were stupid enough to make said city on Hangar and it quickly fell apart to outside griefers raiding said town when we were offline. But that's part of what the game is designed for, if people didn't grief the game would be too easy and you'd get everything far too quickly. So really people like that are kind of required for the game to function right, otherwise it simply wouldn't be as fun or last as long.

And also, not all games are for everyone. Some people enjoy cruel and unforgiving games, others prefer more fair and friendly games. We have a variety of genres and difficulty levels for a reason. There's a reason why games like Skyrim have mods whose sole purpose is to make things harder, or more realistic.

I know this isn't actually what the OP was asking, but I felt this needed to be addressed because it does tie into the players morality. Now to answer the OP more directly.

There are times the person is really just trying to be an asshole, but there other times there is no ill-will meant. That's just how the game works. And also to note, it is a video game and not real life. People play things in games they would never do in real life. Just look at GTA, killing countless innocent people. Millions have done that in GTA, but almost no one has done that in real life.