I am so sorry this happened, I am sure you are feeling horrible
My poor Benji!
I think it might be a sign of him probobly being jealous about the fact that you were getting also close to the other puppy....It was just a reaction and you must work on that slowly, try to associate him with as many dogs as possible. Your Benji loves you and wants to please you. He wants your love. The key words are Patience,
Persistence and Praise. I am so sure that Benji can
be transformed into a peaceful, well-behaved pet."
If you would like to reduce the agression there are ways to go about that too. By not rewarding the bad behavior by laughing or getting excited when he does it. Also by rewarding the good behavior (not growling) with affection and praise. Dont get scared instead be firm with him and let him know that makes you upset.
I wish you the best of luck. Yorkies are wonderful dogs