Even in his last years, Grandpa had a mind like a steel trap. There was no way to confine him to an institution; even St Mungos despaired of keeping hold of him. Equally, we couldn't let him wander
loose. After all, he might find a mirror, and once he saw that he was ugly enough to scare a hag, the future of the entire planet would be in danger.
The only one who could calm him down was Mother. When she wasn't too busy, she kept up a glamour on him, so that he still looked like he had at his prime, before all those awful years in Azkaban had
taken their toll on him. It was worst when she had to go out of the country, to present some paper or other at this or that international wizarding conference; then Father had to 'take care' of him,
which mostly consisted of constantly trying to distract him from trying to find a mirror.
Yes, Grandpa had a mind like a steel trap, but it had long since rusted shut. He still thought that Voldemort was in the ascendancy, rather than reduced to his constituent molecules and scattered across
the four seas in an ancient ritual to purge his soul from the mortal plane. His pureblood rhetoric was a great embarrassment to the family, especially considering Mother's parentage and all, and it only
got worse as he got older.
Yet he stubbornly refused to die. In the end, we all suspect Father killed him. But we wouldn't like to try and prove it. It wasn't as if we liked Grandpa anyway. And if he can kill his own father, we
wouldn't put him past a little child slaughter either. He doesn't want to end up like Grandpa, after all.