Yes, it may be soft, but the key question we should be asking is whether a Puffskein feels resilient or not. For all we know, underneath all that fluffiness it might feel exactly like a rubber ball! Look at the description of a Puffskein, for instance:

Puffskein: The puffskein is spherical in shape and covered in soft, custard-colored fur, it is a docile creature that has no objection to being cuddled or thrown about. The puffskein is a scavenger that will eat anything from leftovers to spiders, but it has a particular preference for sticking its tongue up the nose of a sleeping wizard, and eating their boogies.

Notice the words "has no objection to... being thrown about". Fred may well have seen it being thrown about by Ron, decided that it was a pretty resilient animal and felt no pain, and thus decided to use it in the manner of a rubber ball. A perfectly understandable mistake to make if you consider that Fred may not have seen a Puffskein before and didn't know that it could withstand hand-to-hand throwing/catching but not a whack with a bat.

We don't know all the facts in this case, so I would suggest it is a bit much to extrapolate "vicious animal killer" from the meagre evidence available.