DD .. I use weedkiller when need be, it is a whole lot stronger than the one you used. I doubt I have killed any cats when applying it and I doubt very much that you killed next doors cat either. If it killed next doors cat, then why is your drive not littered with dead bodies of every other cat in the neighbourhood.
Weedol Ultra is a systemic over the counter glyphosate based weed killer. which means that it is fairly safe to use in and around the home .. that is exactly what it is sold for.
The one thing that would raise any concern in using it at this time of the year, it is a bit too early and the weeds have'nt really got growing yet. With it being a systemic killer, the idea is that it is used on a dry day, preferably in 2-3 months time. The spray lands on the leaf and is drawn down into the plant where it kills the root and stops it from growing again. Any overspray from a systemic killer becomes inert when it falls on any other surface other than the leaf.
The fact you sprayed at this time of the year means the spray would become diluted on the already damp surface. It would have been so weak it would have killed nothing.
Been called out to collect dead foxes on a number of occasions, the strange coincidece was they were all on peoples drives. My guess is that any animal that has taken a turn for the worse heads for shelter and it just seems that peoples drives are a poular choice.
Think about it, what do people normaly keep on their drives ... under a car would be a good place to shelter perhaps !