The gerbil bites the dust

[Belle1939]

My cat has opened the door to my boyfriends gerbil cage and attacked the gerbil
My boyfriend is crying on the sofa and won't speak to me and Says it's my fault as it's my cat and the cat is malicious
Aibu to expect him to understand that it's just why cats do and if he had secured the cage properly it wouldn't have happened

err maybe be a bit more comoassionaye - like a proper grown up

Your cat is a cat doing what cats do. While you can say you're sorry it happened, you can't exactly apologize for the cat.

My cat killed my gerbil :-( He learnt how to open the cage and got the gerbil out several times to play befor she died (of shock or a hear attack) poor gerbil.
The cat used to lie next to the gerbils cage purring and purring and purring.

If it was just his gerbil then it was his responsibility to ensure that he/she was safe from the cat, whether that meant simply securing the cage or putting the cage in a room or area that cat couldn't get to. You can't be responsible for your cat doing what cats do.
I keep my mouse shut in a room away from my cats so that there's no possibility of the cats being tempted.
Be sympathetic to him for his loss, but don't let him blame you for something you had no control over

My cat killed one of my hamsters when I was younger (his name was lucky- the irony ) so I've learned to never take chances since then, it's not worth the risk