Cause your player character is supposed to be cast as a good guy. No reason you can’t roleplay as inquest or cast your character as such outside of the game’s story, though. Not every character can be a hero. I view my guardian as a peacemaker and my thief is of pirate heritage, so yeah, I don’t follow the official story/make my own characters heroes either.
I think you might have your conception of the inquest just a little bit… incorrect? Inquest don’t really “kill everything they see”, their primary objective is the collection of data/undergoing of research without any kind of moral or ethical constraint, not explicitly going out of their way to sow the seeds of chaos, so to speak.
As for alignments and stuff, Jeff Grub, the creator of GW2’s overall story, says that the Inquest are actually lawful evil/lawful neutral.
“The Inquest has come up with a way of reducing that loss of signal and what they have done is to effectively create a corporation. They have created a hierarchy, they are much more lawful than the members of their race, but it tends to be lawful-evil, as they are cut off from morality at that point. So they will do things that most Asura will not want. Most Asura would not want to conduct experiments on sentient races, whereas the Inquest don’t have those qualms – they believe the end justifies the means.”
They play politics and have a lot of members in the council and do what they can to manipulate the system rather than disregard rules outright (which is what the chaotic alignment is mainly about).
TL;DR: No reason you can’t roleplay inquest.
