Predator dog retraining

This was a hard day. This morning the cats had something cornered, something that was making a lot of noise. My little chihuahua mix, Gus, immediately ran towards it. So did my GS Lupe. Near as I can tell, Gus got between Lupe and the cats - Lupe pounced, and killed Gus. She then ran inside the house, and tried to hide under a chair. It was pretty awful.I don't blame Lupe - lately I'd seen her sort of lunging at the cat, even running at her a few times, but I didn't correct her as forcefully as I might of. I fell down on the job and poor Gus paid the price. Any suggestions for retraining my 2 year old GS would be much appreciated. I plan on starting on square one and really hitting the obedience, especially leave it, come, and stay. I know she needs exercise - should I exercise her separate from the 2 other dogs or let her join the pack as usual? All advice will be considered! Thanks so much.