Rooster has started harassing broody with chick.

[Morrigan]

I wouldn't kill a proven rooster in preference to an unproven chick.
Do you know the sex of the chick, or what breed it is?

I'm more concerned about my proven broody, to be honest, along with future broody hens and their chicks.

 

[Shadrach]

I'm more concerned about my proven broody, to be honest, along with future broody hens and their chicks.

Yep, I can understand that. I would take the risk. I can understand why others might not.
I can't speak from experience because all the chicks hatched here are the progeny of parents that from a particular tribe here. However, as you must be aware, for many with contained chickens, having a rooster that has been 'well behaved' with mums chicks and humans is a bit of a result.
As you have doubtless read, you can pick any old chick out of a bargain bucket at your local farm store.
6 years old isn't really very old for a chicken with good genes. It may be a good age for hatchery stock.

 

[Chelsa'sChicks]

It may just be something between him and that one hen, I don't know. But yes, I would not trust him with a broody and chicks. You can isolate him from that hen and chick if you want, he may be OK with the next broody and chicks. But I'd look for a replacement rooster.

I wondered this myself too.. Is she a hen lower on the pecking order?

 

[Morrigan]

I wondered this myself too. Is she a hen lower on the pecking order?

The hen in question is a two year old speckled sussex, and a mid-packer in terms of pecking order. She went broody last year as well and raised a group of chicks in the main coop with no problems or interference from Duke, or from anyone else. Although Duke's ire increased when she puffed up to defend her chick, she was sitting in the corner minding her own business when Duke first went after her. Now she can't stand the sight of him and, without the hardware cloth separator, I'm sure they would have gotten into it again.

I have another hen broody and due to hatch this weekend. I have no confidence that Duke will treat her any better and am reluctant to give him that chance.

As you have doubtless read, you can pick any old chick out of a bargain bucket at your local farm store.
6 years old isn't really very old for a chicken with good genes. It may be a good age for hatchery stock.

For me, it's not whether the chick has a high monetary value, but rather that I don't want a rooster who would injure or kill a chick. However wonderful he has been in the past, he clearly is no so wonderful now

I wouldn't have thought 6 years that old either -- my 6 year hen is still in vibrant condition. But, something is definitely not right with Duke. He has slowed down and has been a bit crankier with me lately as well.

 

[Morrigan]

We did the deed last night. We waited until dark, and then dispatched him before he even knew what was happening. It was my hardest chicken keeping decision yet and I'm still teary-eyed over it. It came down to the fact that I didn't have a good enough way to separate him and I no longer trusted him around my broodies and their chicks. Maybe they would have worked it out, or maybe I would have come home to dead chicks and an injured hen.

I might have been willing to take that chance had he been younger, with solid genetics and in great physical shape. But he had definitely lost a step and his genetics were always iffy -- I became increasing certain that he carried the tufted gene defect -- as I could not hatch eggs from his daughters due to high chick mortality. Had my replacement rooster worked out last year, Duke probably wouldn't have made it this far.

I'm comfortable I made the logical decision, but it was still very emotional for me. At least he had a good, long life. He outlived all but 1 of my original 15 chicken flock. RIP Duke.

 

[aart]

I'm comfortable I made the logical decision, but it was still very emotional for me.

Condolences and Kudos.
It is the one of the hardest decisions.
But the peace of the flock will help soothe the tender feels.