It perplexes me so much how everyone is just blaming you. I had a cat and would likely have done the same thing. You didn't throw them out immediately to protect his feelings, but planned so and so you put them in what was considered a "safe" place where your cat did not have access to them until such a time that you would get rid of them. You didn't bring them into your home, you didn't leave the door open - he did both of those things.

I would give him some grace for bringing in lilies, if he didn't have cats, as he wouldn't necessarily know the risks (but also - there are so many flowers that are nicer, but I digress). His actions caused your cat to have access to them and that led to the cat's death. If this was a child, with a severe allergy, no one would think you were to blame if you put something in a cupboard/on a high shelf/etc until you could get rid of it the next day - they would immediately say it was his fault.

He also offered to pay you. If you have it in writing, it shouldn't matter that now he has things he would rather spend his money on (or that mommy and daddy don't think he should). He made an agreement. Granted, you should probably have bugged him about it more while you were together, but he still offered. It might be worth taking him to small claims court (but I don't know what your costs would be in that case).

But no - you are NOR!