I think you're mistaking the term snap for just getting tired of handling someone's needs. If you kill someone just because you don't want to handle the care for them, it's 100% opposite of snapping because you've reached the limit to which you CAN care for them. (Not want to, but ability to.) You don't snap based on a cognitive rational thought - you snap when you overload your being.
If you were a member of my family, and I cared about you, I promise I'd only kill you if I wasted away to a mere nothing financially and physically first and would have to be in a hospital for the criminally insane afterwards because I took care of your needs to the detriment of my own.
If I didn't like you, I'd let you live in agony and collect your social security checks.
Actually, the only person I'd give my life over to 100% is my daughter. Everyone else can go to a nursing home, where they don't snap because they don't care.