I guess it could be possible but grocery store roofs are pretty high, 25-30 feet?
I just don't see it happening. Afterall, he had just killed the lawnmower victim about 45 minutes earlier from all indications he was in a vehicle and the Leisure World victim (25 minutes later) had to come from the parking lot as there are no nesting areas beyond the parking lot. The only buildings in the parking lot at Leisure World, I believe, is a bank and a fast-food joint. Banks have security that would detect somebody on the roof and fast-food places are too busy to get on and off without being seen.
So I don't think this guy was a "sniper" per say, he had a mobile platform and took the shots when the opportunity presented itself.