Sudden and severe aggressive behavior.

[Riptide]

It might be her thyroid. Thyroid problems are common in Beagles, and seem to pop up in a dog's senior years. Thyroid problems can cause behavioral changes so I'd get a test on her ASAP. Meanwhile I would keep them separated to avoid injury.

[conchitaroja]

She killed a squirrel a few weeks back? is it possible she was bitten by the squirrel?
What does anyone out there know about rabies, is it transmitted only by being bitten by an infected animal & not biting one? How can a dog be tested w/o killing it?
Oh, I pray she's ok & that this can be easily treated....
good luck to you

[Riptide]

She killed a squirrel a few weeks back? is it possible she was bitten by the squirrel?
What does anyone out there know about rabies, is it transmitted only by being bitten by an infected animal & not biting one? How can a dog be tested w/o killing it?

Killing the squirrel is a prey drive thing. All dogs could do it, regardless of how sweet or aggressive they are. Its just plain ol' instinct, not a matter of meanness. Chasing and killing prey is a fixed action pattern - it's as natural to dogs as breathing is to humans. This sort of behavior isn't an indicator of aggression, especially since she's a Beagle (hound - bred for hunting!).

As far as rabies goes, I highly doubt that's the issue. I don't believe it can be transmitted by biting an infected animal. My understanding is that the infected animal's saliva has to come into contact with the other animal's blood. There is no way to test an animal without killing it. That being said, again it is HIGHLY unlikely that's the problem considering the circumstances and that rabies is uncommon in pets these days because of vaccinations and whatnot.

[*LabLoverKEB*]

Here is a quick overview of rabies:
Rabies is an often fatal disease, shed through the saliva of an infected animal. Animals acquire rabies primarily via bite wounds from infected animals. Squirrels can carry the rabies virus. All mammals are affected by this disease. The rabies virus effects the neurological system. Neurological symptoms include aggression, depression, paralysis, seizures, excessive salivation, inability to swallow, and eventually death.
Diagnosis for rabies includes quarantine for 10 consecutive days to observe for above symptoms. Like Riptide mentioned, the only reliable test is the microscopic examination of the brain tissue. (By cutting of the head). Unfortunately there is no treatment for animals with rabies. But as far as prevention goes, there is the rabies vaccine. (There is even a rabies vaccine for humans, but the side effects make you really sick. As far as I know, all licensed veterinarians are required to get the rabies vaccine.