Just a thought, but the key here is probably that "you have had most of them fixed". A male cat who is neutered has no need or drive to kill kittens to bring their mother back into heat (sadly a well known male strategy known as "infanticide" in many animal species) because he is not driven by hormones to reproduce at any cost (this is why male cats fight so much if not neutered). A male cat who is father or close relative of kittens probably will not hurt them either, provided he has been around mom enough to recognize himself as a possible father, indeed as others have posted, he may actually help with raising them, and will likely drive intruding males away as well to protect his reproductive investment. The danger is when an unrelated, intact feral male moves into the area where a mother has vulnerable kittens. It is their instinct to hasten mom into heat by killing the kittens. Terrible thought that seems, remember we cannot judge by human moral standards, they just don't apply to the wild drive to survive and reproduce. (Important note: I'm referring to true ferals, not stray house cats). The best solution whenever possible is to TNR feral male cats; the second best is to provide an environment where they cannot easily intrude which yes, includes having neutered adult cats to help keep them away. I've seen our Mama cat Patience and her son Patches join together to face and chase off not just other cats but LARGE DOGS to protect their kids/siblings! We don't have intruders on our property any more, other than a neighborhood cat that Patches has befriended and keeps trying to bring inside!