Oh WhoDat, this is a horrible thing. My heart breaks for you.
I know the replay you are doing as I have done it with a tragedy here, and it does not help to berate yourself. Try to forge ahead if you can. Because you need to survive it too, and your mind is not always being your friend by putting you through the imaginary scenes it makes up and replaying what you did see. PTSD is real and can become a problem if you're unable to process through the horror.
Your dear chickens that made it through do need you now and doing what is needed for them is your path forward, for you to save yourself too. Try to stay in the present with them. They aren't running the tape again and again, they are trying to calm down and feel good and get through the day too, they have to metabolize the fear chemicals generated, get a sense of safety back and have their routines resume. You do too.
OK sorry to belabor the point but this is a real trauma to you also, you know this. So you need to treat it too. This may help - when you feel the horror, let yourself feel it but don't run the imagined scene or the picture you got when you discovered them in your mind, that prolongs it, enlarges it, and you can get stuck there. Just stay with and acknowledge the feeling that has arrived, staying aware of your present surroundings, with the real things and beings around you. Be with your pets, your chickens. Let the feeling come but also let it go, let it pass through you and it will hopefully slowly diminish and you'll be able to breathe.