Salarian informant gave them the info that cured krogan female was at the STG base. TIM knows that her dying would cause intense heat between the turians, krogan, salarians, and Alliance. Them being at each other's throats weakens Cerberus opposition, TIM has more time to develop troops and combat reapers his way.
I guess this was TIM's reasoning but at this point, his mind must have already been seriously messed up by indoctrination because this kind of reasoning is a mess:
- As stated earlier this only makes sense if Cerberus is building their own crucible. If they are not, then they should be just as interested in making Shepard's plans work as Shep him/herself. After all, TIM acknowledges that he needs the crucible to control the reapers.
- If TIM wants to build a second crucible only with Cerberus recourses, he has even more delusions of grandeur than we already thought he has. It takes the efforts of an entire united galaxy to build the first one. I get that Mac Walters loves Cerberus and wanted them to be on par with everyone else but this is a bt much.
- Even if we assume that he can build his own crucible, it is questionable that sabotaging an alliance between the frogan and the turians is in his best interest. After all, the biggest single power in the galaxy at this point are the reapers. They are the ones who need to be opposed and that works best if Palaven holds out. Also, there is nothing to be gained by TIM from having Palaven fall. The turians are not planning to hunt Cerberus when given a respit from defending their home world. The Primarch only offers help once we deploy the crucible. The reapers on the other hand seem to oppose Cerberus directly (see Sanctuary).
So apparently the reapers cranked up TIMs indoctrination to prevent him from thinking straight, which is a bit cheap writing but not implausible. After all, TIM's mental decay clearly already starts to progress from the beginning of ME3 onward.
One thing I am curious about though: What is this talk about the Salarian insider that Cerberus has in the STG? Is this actually mentioned in game (can't remember)? Because I would have to ask: What could possibly motivate a salarian to betray the entire female krogan issue to Cerberus of all people? Mordin (or his substitute) was the informant for the krogan and he had his reasons, which are explained but Cerberus? I guess that salarian must have been indoctrinated as well?