I am raising a stray cat.It gave birth to four lovely kittens.A male cat killed them,slashing their neck.Why??

I have a stray cat which had a litter of four lovely kittens.We kept it in the loft bcz it was very roomy & comfortable than our living room. But a bug male cat,which I suppose is the father of those kittens,killed all four of them . As if it is professional killer, it slashed their necks . Not even a sratch was found on any other part of the body.My cat is still upset & meows everywhere and is searching for her young ones in every nook and corner.It is such a pitiable sight. Can someone explain why the male cat did this please....i am still haunted by the sight of those dead kittens.