Sorry I misunderstood you.

So far as I can tell, the main issues with execution methods are that execution methods have to fit into canon, thus the canon-appropriate weapons; or the portaling of Sues into dangerous locations or within reach of dangerous creatures. Portaling Sues into other continua for execution is generally frowned on but still done occasionally.

Torture for torture's sake doesn't fit into most canons; and where it does, it tends to draw attention to the Sue--which is the opposite of what's desirable. Humiliating the Sue makes sense; karmic or ironic deaths make sense; but torture just makes her more sympathetic, which is a Bad Idea. The purpose of a mission is to restore canon, not to get revenge (though agents understandably forget this more often than not!).

Torture can be physical or mental. I think, for example, that somebody on the Wiki mentioned the possibility of disposing of a Sue via the Jaunt--which is torture by any standard despite the lack of pain. Choosing a painful death, especially by canonical methods (like Mirwood spiders), is not forbidden. Deliberately causing more pain, however, is frowned on (for example, shooting a Sue repeatedly in not-immediately-fatal locations is torture; shooting her once and letting her bleed to death is not).

I know--it's a thin line. But hey, they're assassins; what do you expect?

... for finding, er, questionable methods which hadn't been specifically banned yet and using them. Monsters which rape their victims to death are apparently frowned on, as L&F discovered after the incidents in question - one with the Redneck Trees, one with Drip Rat. They claimed they fail to see how it's that much worse than being eaten, but still got in trouble.

(I understand why it's frowned on. The characters did not.)