We listed over a dozen reasons for trolls to hate Star Citizen in a discussions thread on Massively Opinionated once but I don't remember all of those reasons from the top of my head ATM.
However, IMO it's the sum of them which makes some people go overboard. Most people are used to shallow one-genre games with a clear end objective, which is reachable within a couple of hours, played on their console ... Star Citizen is none of that. And that pisses people off - "Who do you think you are, telling me I'm playing shallow games on an inferior platform?" Of course, no-one are telling them that, but they get the feeling that's what many people think.
So, basically, Star Citizen is so different from everything else they know from before and have played earlier that they get into a defensive position in a trench.
But you're right, there are probably some competitors trolling too.
Yep. This was one of the original stretch goals for the project as far as I can tell.