This cat was about to become a father .

But he still would. The only reason they attack the kittens is to mate with the mother again.

Yeah you're right

But doesn't that only happen when they're not his kittens?

Yea. They won't kill their own offspring.

Yes he will. Especially if there is the scent of another male around he will assume the litter isn't his and kill them.

They know what kittens are theirs.

It depends on the tom. Tom's mate with a female and immediately move on to the next one and rarely see the litter they sired, but if a Tom is around the female he impregnated then there is the risk that he will kill them.

http://pets.thenest.com/male-cats-act-paternal-fatherly-towards-kittens-9281.html

Soooo we were right. Tomcats don't *usually* kill their own kittens.