You don't need to tell me this but think, why do you want that rooster? What are your goals and how does he fit in? How hard do you want to work to keep him? Why would you not want to put him in the freezer tomorrow?
How long has this behavior been going on? Was that hen the dominant hen? How old is that Sebright, I'm not sure how he fits in. Who is dominant, the Rock or the Sebright? How are the Sebright and the Rock getting along?
I once had a cockerel that was raised in a flock without a dominant rooster as I removed the dominant rooster when the cockerel was about 5 months old. Until that cockerel was 11 months old the dominant hen ruled the roost. The other hens would submit to him but if that dominant hen was around she would knock him off. He was pretty much a wimp. But at 11 months he finally matured enough to try to take over the dominant position in the flock. For two days he was absolutely brutal to that dominant hen. He would force her to mate. Out of the blue he might attack her and peck her around the head, which is a dangerous place for him to attack. After two days she accepted his dominance and they became best buddies.
Something has caused your Rock to change his behaviors. I don't know if it is anything like my scenario above. I have seen one chicken to just all of a sudden take a strong dislike for another chicken and try to kill it. That could be going on. By the way, that cockerel above never did become a good flock master, his personality was too weak. That often happens with cockerels that are late maturing.
If that Sebright is older and the Rock is a late maturing bird, you might see some trouble between them. Even with the difference in size it is possible that Sebright has dominated him so far.
Unless I had a strong reason to keep that rooster I'd put him in the freezer tomorrow. If you really want to try to keep him you can try isolating that hen from the flock for a week, then put her back and see what happens. By isolate I mean totally away from the flock.
