Truth and Lies remember to ask OOCly before. Because really infiltrating a guild is just as bad as infiltrating a guildhouse 
Sounds like somebody is scared 
And no, not really. Infiltrating a guildhouse is generally bad because of game mechanics. Infiltrating a guild is all RP. Ain't nuttin' wrong with that.
Taking leadership and disbanding the guild though, would be uncool. Running it into the ground through RP, though, is fine.
No, some people might find that grieving. I am sure those PLer guilds won't like it.
And? Who cares about PLer guilds. If somebody RPs infiltrating a guild and does it well, then there is nothing wrong with that. As I said, if he uses game mechanics (i.e. disbanding the guild) then yes, that's grieving and bad, but besides that, I don't see anything wrong with it.
RPers who run off the second something bad happens to them are no better than PLers. I remember almost killing Zorbels and she never complained once. She was a good RPer. And horrible things happened to a few of my characters and I never ran away, just tried to use them to my advantage.
I agree that it's a game and people should have fun but, if you're not letting anything happen to your character or guild without OOC consent then you're ruining the fun for all the spies/assassins/thieves out there just as much as people who abuse game mechanics. Have fun with your guild surviving being infiltrated, or make surviving post guild destruction into a cool RP with revenge and what not. You're basically writing your characters story and who hasn't had some hardships in the past?