My German Shepherd injured my Lhasa Apso,?

Hello, Just wanted to know what our family should do. Our 2 yr old German Shepherd has tried to attack our Lhasa a total of 6 times. 3 times we were able to grab her before she was able to put him in her mouth, but the other 2 times she got him and we did get him loose. The last time she got to him, d his abdomen and we had to put him to sleep. We fought with her to get her jaw open but nothing worked.My son finally got her to drop him, and we rushed to the hospital. I don't understand why she acted this way, we got her at 5 weeks old and he was 5 yrs old, when she turned 2yrs thats when it started with the problems. We had her in training at 7mths and she did well with other dogs, and have taken her to dog parks and 2 times she growled at 2 dogs there. We had them separated all the time and then worked with the trainer again with both of them together. The trainer never thought she would hurt the lhasa, said she wanted to play, But we saw if differently. We thought if she ever got the chance, she would kill him, and thats what happened. She has never been aggressive to us or any other human, but she did cause injuries to my son who was bitten several times trying to stop the fight. Because she did cause the injuries that killed my Lhasa, what should I do now. Please let me know what you think? We are very saddened .