People tend to dislike a person who is productive without being productive in a way that directly benefits them, the outsider. This is especially true with collaborations, in which someone who is productive outside of the group whilst not being as productive within the group is detested upon greatly. I have been on both sides and find that, even without action, every route involves increasingly negative emotions the longer the project is continued.
To the outsider, there is a feeling of betrayal. It feels as if that person simply does not care enough for the group, your group, to do their duty before taking care of their own needs. The extra activities are seen as inferior to your own group's work, sometimes to the point where the extras are seen as mere hobbies, relaxation activities, while the work is seen as genuine work, regardless of how it actually is. To confront them is to risk losing a friend, and to many there is a certain guilt rippling through them as they are forced to place blame on said friend. To remain passive is to continue watching as this person continues on with their ways, the negative feelings bubbling inside you, boiling in a discreet heat. A hate begins to form, and sometimes there's guilt in such a harsh emotion being formed due to such a reason.
On the other hand, the other person tends to have just as much guilt. In my situation, I find myself unable to do any work at all. Guilt crumbles around me, threatening to crush me no matter where I turn. It's hard to breathe, and I'm trapped, I'm trapped with no where to go, no action to take to save me from this predicament. Any attempts to work on the project itself lends guilt in the form of "Why didn't I do this sooner?" "Are they going to use this against me? 'So now you do it, huh?' 'Why didn't you do this before?' or 'Where have you been all this time? Y'know most of this is me. Why do you still bother?'". Anything outside of the project brings up the guilt, the "Why aren't you doing what you should be doing?".
Of course, both of these are simply how I felt in each situation, and imply that the person has a certain sense of remorse. Certain people lack these feelings, and while they may be hated more so, I kind of envy them for their ability to continue on unaffected by such things. Of course, I say that about any kind of ability to disregard feelings, so please take that lightly.