Great news your pup was about 7 weeks old when you got it, that's a help.
There is no 'sudden' about these attacks. It's happened 3 times and it's an issue that must be addressed. As you've seen, these fights can be serious, even deadly.
Management is key for the time being. Always you are with the beagle so you can protect and not let the bully GSD keep the fear/stress going. Not fair to your older dog if you allow a bully to take advantage. As the 'boss' in the house, you have to step in BEFORE the fight starts. It's not up to your GSD to start a fight like the decision is theirs. It is YOUR house, you are in charge and there are behaviors you should expect and hold your dog to.
In the mean time I'd WAY up the exercise for the GSD. Outside the house. Swimming, running, hiking, ball chasing, socialization, hours of exercise and MENTAL stimulation outside the home. Give that GSD something else interesting in it's life besides finding an opportunity to pick on a smaller dog.
And dog obedience classes. These classes are NOT about the perfect sit or down (though that's an added benefit). And it doesn't matter that my dogs are perfect when alone in the house in the kitchen. Are they also just as fast and obedient when we are out and about in front of the Walmart? Or are my dogs (so perfect at home) suddenly unfocused and apparently deaf?
For my dogs to REALLY learn and REALLY listen they have to respect me as a leader and behave in spite of distractions. The best way I've learned to do this is have a professional teach me. Only about 25% of dog classes are about teaching the 'dog'. The other 75% is about teaching ME to teach my dog. When I do it right, my dogs learn (go figure) and when I don't they keep doing the same BAD BEHAVIORS they've been doing all along.
Dog aggression is serious and we must act pro-actively to prevent future problems. They don't get better otherwise. Only worse. But I've found when I DO make changes in the home and in MY behavior and training (dog classes) my dog's also change. And for the better.
Here's some stuff to read that may help while you are searching for those good classes:
Showing your beagle that you are taking charge and going to step in to stop the bullying is important. So this leadership is important for BOTH dogs: