German Sheppard killed my pug?

I left home Saturday and returned Sunday finding that my 12 month old German Sheppard killed my 10 month small pug. The Pug would always growl and played rough with the Sheppard, take away treats or food from him. The two dogs would sleep and play together all the time; however two weeks ago everything started to change the Sheppard would attack the pug this of course without hurting him and the pug would freeze. My husband told me that the pug had bites by his arm which he thinks it might have been close to his heart. My question and concern is that I’m scared for my 3 year old son, since he would always pull on both of my dogs tails, ears and they never did anything to him but with this happening I don’t know what to expect…