I think my rabbit has started killing some of my chickens.

[El - Ahrairah]

A male rabbit lol!
I’m not sure if it could be a wild bunny from where you are, I know that the US rabbits can’t breed with the Europe rabbits (domesticated ones same as EU)
I thought maybe a male wild rabbit could be coming for her at night or in the evenings and her desire to mate could be driving her mad? rabbits (EU) are also group animals so being 100% alone is usually stressful for them.

If she had full access to the coop at any time I wouldn’t put it past her to kill a chicken but the part where she brought it back is interesting in the least! I would buy a havahart trap just in case to be positive it isn’t a small predator. I’m not sure of what predators you have there, but it’s always a good idea to be on the safe side!

I’d just keep her away from the chickens the best you can now. Maybe if she doesn’t already, give her a few feet of run space with small hole chicken wire on the bottom. I had my rabbits in a “chicken tractor “ and they loved eating the grass. I eventually put them in a hutch though.

The male rabbit theory is interesting...

Where I'm from, there are three types of lagomorphs: European rabbit, brown hare and mountain hare. One of my neighbours has a male rabbit too. Yes, my rabbit is an EU type (I live in Europe too).

I've read before that female rabbits tend to be bigger and more aggressive than males, and it could be made even worse when they're in heat.

Not a bad theory, actually.

I checked for signs of predators before and after the incident, very carefully too. Everyday I tried to inspect the garden thoroughly to see if there were predators or any signs of them (e.g. footprints, nests, dens, etc.). Nothing. Just to be thorough, I didn't just check the yard but also in my house and the environment nearby in general. No bins were tipped over outside, no other footprints, etc. The kills weren't clean or reminiscent of a predator, either. Many people have considered weasels to be the suspect, but I don't think so (no footprints, no harm done to the rabbit, wasn't a clean kill (there was blunt trauma on the bodies and injured chickens, scratch and bite marks all over), no sightings, etc.). With that being said, I'll look into making the coop more secure in case of another incident like this.

Wait, "zombie raccoons"?

 

[kittycat1356]

The male rabbit theory is interesting...

Where I'm from, there are three types of lagomorphs: European rabbit, brown hare and mountain hare. One of my neighbours has a male rabbit too. Yes, my rabbit is an EU type (I live in Europe too).

I've read before that female rabbits tend to be bigger and more aggressive than males, and it could be made even worse when they're in heat.

Not a bad theory, actually.

I checked for signs of predators before and after the incident, very carefully too. Everyday I tried to inspect the garden thoroughly to see if there were predators or any signs of them (e.g. footprints, nests, dens, etc.). Nothing. Just to be thorough, I didn't just check the yard but also in my house and the environment nearby in general. No bins were tipped over outside, no other footprints, etc. The kills weren't clean or reminiscent of a predator, either. Many people have considered weasels to be the suspect, but I don't think so (no footprints, no harm done to the rabbit, wasn't a clean kill (there was blunt trauma on the bodies and injured chickens, scratch and bite marks all over), no sightings, etc.). With that being said, I'll look into making the coop more secure in case of another incident like this.

Wait, "zombie raccoons"?

https://www.backyardchickens.com/threads/rabbit-killing-chickens.116708/page-2

A few people have had the same problem as you.

 

[Hyroler]

Sounds like a job for CIS Europe to investigate.Maybe even make tv episode about it.