Here's the general idea of the real mindset of these players:

I kill them because I am level 50 and don't care about experience points. I am doing the fate for the Seals and you poor Fate Grinders are wasting my time to quickly gather an easy 1k seals because you want to farm some crappy mobs. I will continue to burn down every lamb I see in that fate and laugh at all the non 50s who complain about it. Go run a friggen dungeon, Aurum Vale is an easy 80k-100k xp.
I just do it as intended by the FATE. If someone in shout chat asks NICELY, I might acquiesce. If someone in shout chat starts immediately by writing "OMG DON'T KILL THE LAMBS, N00BZ!", I kill the lambs while savouring every second of it.
... I like dis...
*Slow and evil smile forms.*
It's funny to see how upset people get over a video game therefore I will kill the lambs and then laugh at them.
Now I'm convinced that this has less to do with "completing a FATE as intended" and more to do with people trolling others... because they can.