Mixed Breed Hound Agressive

We adopted a mixed breed hound dog, a female,  at 7 weeks old from a kill shelter.  She is now 1 year old.  She is spayed and micro-chipped and well taken care of.  She weights 54 pounds.  She loves our mixed breed male dog, almost 3 years old.  She even tolerates our cat.  She wants to hunt for sure.  We have almost 1&1/2 acres fenced in to keep her in.  She lives inside but has an electronic doggie door to come and go as she pleases.  We have never seen her aggressive except when a ground hog got inside the fence and that  baby lasted about 30 seconds.  She killed it before I could get to it.  Then today, our neighbor   was walking his very docile part pit bull female close to our driveway.  He came over to say hi and my hound went wild.  She wanted to kill that neighbors dog and the dog was being very friendly.  It shocked my neighbor and me.  I have cried all evening.  Now, I am afraid she will hurt my other dog, even though they love each other dearly.  I can't trust her.  I take her to the vet tomorrow for  immunizations and a physical.  But I want to talk to the doctor first to see why my hound, Grace, went crazy on that female dog, to kill.   My neighbors part pit dog has also been spayed and is 8 years old.  Why did my hound dog all of a sudden turn aggressive?  What can I do?  Thank you! 

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