What's stopping the Secret Service from killing the President?

I can't think of any other people that are always close to the President and carrying guns. Now I know that these people are trained extensively to take a bullet for him, and have to go through rigorous background checks to even think of getting close to him. But what if they become corrupt? What if someone has a really good reason to kill him, and uses it as the perfect opportunity to do it? What if somebody gets into the job for that very reason alone, and waits 10-20 years just to get the opportunity to fire a shot? Of course the President probably won't be the same in that time, but they might just do it for the sake of killing him. Of course he'd probably be shot to death by other Secret Service within minutes, but some people are martyrs. If there are so many Secret Service workers, what's stopping only one of them from committing the unthinkable?