Why did our Dog kill 2 pups

Question
Hi, In the last 2 days our dog has badly hurt 2 of her 10 pups causing us to have to put them down.
Shadow is 1/2 Lab, Husky & Shepard.  She is a year old and this is her first litter. The remaining 8 pups are 3 weeks old.
In the last 2 days she has started attacking her pups.  Nothing out of the ordinary has happened to stress her out.  Enviroment is the same.  
For surivival of the puppies we have moved them into the house...we are still letting Shadow nurse them 4-5 times a day with supervision.  Starting them now on baby pablum.

I am just wondering why she would attack her young after 3 weeks, also what should we be doing with the remaning pups? Are we right in taking them away?  

Thanks for your help!

Answer
I love dogs and am willing to give hours and hours answering questions from owners that will help the dogs.  I am very concerned about the slaughter of dogs for lack of homes. I am afraid many of the questions here are from poorly informed people breeding dogs.  I can't in good conscience facilitate that.  Go to www.akc.org and read about responsible breeding.  Go to www.petfinder.org and see how many dogs in your area need homes.