I agree. I think it is relatively common for the aged to become violent. I think age-related illnesses or disorders are often the cause, much more than perhaps the general public is aware.
Alzheimer's (though not apparently affecting your grandfather in that way), Parkinson's (late stage with dementia), strokes, and other disorders can manifest paranoia and tendencies toward violence in the elderly. Both of my grandmothers were so afflicted. Fortunately, my maternal grandmother (who suffered from dementia brought on by late stage Parkinson's and believed the last six months of her life that she was being held hostage in a bank by conspiring church officials) was too weak to act on her impulses. She had moments of clarity toward the very end and became highly affectionate in those moments, but was primarily verbally abusive and physically resistant until major changes in her medication. My paternal grandmother had Alzheimer's and suffered Sundowner Syndrome. She became easily and unreasonably agitated in the late afternoon and early evening, highly paranoid, and sometimes physically threatening. She would break things, run away, act out as if she were fending off a violent attacker, and frequently accused other patients of intending her harm. The rest of the time, she was sweet and friendly, adoring of soft, cuddly things living or stuffed, and told jokes.