She spends 25 years coming up with a daft plan to undermine the entire guild because she's been framed by Mercer Frey. She twitch shoots the PC because she "can't get a clear shot" - maybe you should have waited for more than a second before firing then. In doing so she wastes a poison it apparently took her a year to make but never occurred to her to make more than one arrow's worth, nor did the thought occur that paralysis poisons are common and she could have paralysed Mercer and tied him up. The thought never occurred to her Frey might not be alone and have someone with him - the player - he's decided he wants dead for no reason. She declares she's saved your life by leaving you paralysed and helpless, in reality saved only by Mercer Frey's incompetence in making sure you're dead - he could easily have stabbed you through the heart or cut your head off.
Then, after 25 years, it suddenly occurs to her she has G's journal and she could try to translate that. Fortunately, Brynjolf and friends are gullible enough to believe some text they're told is a translation without having any means of checking whether it's at all accurate, so really she could have just made something up on day one. To be fair, they confirm this by checking Mercer has somehow been lugging vast amounts of gold through a room that always has several professional thieves in it without being spotted. Luckily, he's left his plans lying around in a house he apparently never uses.
Then she thinks it's a good idea for the player and Brnjolf to sell their soul for seemingly nothing more than a set of armour probably inferior to what the player already has and this will somehow help defeat Mercer. This resurrects a sub cult in the guild named after a songbird for some strange reason that guards a skeleton key supposedly incredibly useful for thieves but they're not allowed to use it. Unless they're the player, in which case they can carry it and use it for as long as they like. After defeating Mercer in a 'heist' G planned 25 years ago but nobody acted on despite the fact it entailed no more planning than going to a dungeon, something you just go and do on the spot, Karliah and the rest of the guild seem to to just forget about all the treasure Mercer stole in their policy of putting all their treasure in a big communal vault and leaving it there for some unexplained reason. Why the player was exempt from putting into this communal pot, what it's supposed to be for and why nobody had apparently noticed anything going missing over the course of 25 years is, of course, a total mystery. Instead, Karliah tells you she can't possibly accompany you on the final quest, but once you've battled through the dungeon she suddenly appears out of nowhere.