I think I know the guy you're talking about - I see a guy with that description in Peckham park all the time walking a various amount of dogs, usually he walks with a couple on leads and a couple off leads. And I've spoken to him a couple of months back and a few of the dogs he walks I remember him saying were for other people. If it's any help when I've seen him in the park he seems really nice, cheerful and good with the dogs - I've never seen him angry or cruel with them. However, the guy I'm talking about might not be the guy you saw, or if it is the same guy, maybe, as you've suggested he might have been having a bad day. I hope that the case, for the dogs sake.