Grim isn't killing the chickens. They just don't have that long of a lifespan. Unless you turn off animals aging in your game, you'll likely see Grim a lot. Also, one thing that might help is to stagger the purchase of new chickens so you're not losing all of them at once. I know this can be disturbing. Every time I pop into the lot where my Cantrell kids' parents live, it does seem Grim is there. Joseph has a farmhand, too. Best of luck. If you need to know where to turn aging off for the animals, go into Game Options. Click on those little Ellipses dots (three dots in the upper right) Click Gameplay and from there you will find the option once you scroll down the page.