I'm going to let my exes cats die
Like they're sick and you're not caring for them? Or are they just old?
They have been with me for so long and now it's their time to go.
That's understandable, animals eventually die even when we care for them. It is okay to feel sad but accepting of a pet's death.
They are supposed to die soon anyways and I can't be bothered to take care of them.
If they are sick, then you should do what you can to make their death painless. Especially if  you cannot care for them during their last days.