Elements and chemicals are very different things. Silicon is the #2 most common element on earth and if you smash a bunch of solar panels, they turn into sand when they get wet. Or if you just wait.
Even pure silicon metal is totally benign, even in large amounts. Certainly safer than iron, which can kill you in relatively small amounts if it's in a bioavailable form. Silicon is already literally everywhere, in every form it will reasonably be transformed into.
Even the boron and phosphorus that make up a tiny percent of cells are extremely benign. The biggest issues are the tin fingers and solder, and probably the lead that is often present. But lead is also far from necessary. And unfortunately, the human race is going to be dealing with lead dust for a while already, just from how it used to be in gasoline.
1) This is the technical answer: Cyanide binds fairly strongly to one of the molecular machines involved in cellular respiration, inactivating it.
The result of that inactivation is that your cell begins to have issues producing ATP, even when it has nutrients available. Without going too far into detail, ATP is essentially the cellular unit of energy that your cells run on. Without being able to make more, your cell will eventually run out of ATP and it will die. Enough cells die? You'll die.
2) The more important point here is that not only is your intuition uninformed, but you appear to be incredulous as well. Ask yourself why you react to unintuitive results in areas you're not familiar with in the way that you do.
