Well, just keep eating more and more, eventually you'll die from that.
The most expedient means of killing yourself is with lab grade helium. Basically you'll suffocate to death without actually feeling like you're suffocating.
Don't fuck it up though, cause you might just wind up brain damaged instead of dead, so make sure no one is going to try and save you or that the seal on the mask/respirator you're using can't be easily breached (like if you fall over and the mask gets pulled off a bit).
There's also Tylenol, tons of people die from inadvertent Tylenol overdose every year.
Ultimately though, if you're morbidly obese, just go out like Shay and eat yourself to death... but not before shitting all over... everything...