Why is it bad to breathe pure oxygen and is it true it strips down the molecules to a single electron or what?

It can be bad. Air is approx 80 % nitrogen and 20% oxygen. When you breath 100% oxygen it creates free radicals that act like "pacman" and destroys lung tissue. This can occur with in 6-12hours of breathing the pure oxygen.
Not sure. Not clear how pure oxygen causes damage. The air force gave young, fit jet pilots pure oxygen for 24 hrs and measured their lung function, they noticed a 25% drop in their lung capacity. That's most of what we know about this.
Pure oxygen is toxic. Pure or 100% oxygen is toxic or harmful to airway and lung tissues. Pure oxygen causes oxidative injury to tissues and will burn if inhaled. It will also cause cilia or hair cells that line airways to stop working within hours and decreases the rate of lung growth (in infants) or healing by slowing protein production. Supplemental oxygen is very safe when used at 30% to 50% concentration.