I don't think I'm gonna give my roommate the money he needs for his cat's surgery.
Why would you not help save the cat's life?
Because he was always bragging about how he loves to spend money. This is another example of his self-absorption.
Of course you're not responsible for paying your roommate's bills, especially if he wastes a lot of money. But if you have the money, it would be kind to both the roommate and the cat to help save its life.
Fine, I'll pay for it. Maybe I'll charge him more. Maybe I'll never help him again.
That's nice of you to help pay for the cat's surgery. I can understand why you wouldn't want to give money to your roommate if he's a spendthrift, but the cat shouldn't be punished for his owner's irresponsibility.