MSG is most certainly harmful in the quantity we add to food, not in the way rat poison is but in the way obesity is, it tricks your brain into thinking it's eating protein which is a problem since it's usually put on empty carbs like Doritos or noodles a food with it's own problems.

your body naturally wants to eat to much and they are slow to metabolize and we take the fiber out.

All that combined with the fact you think you are satisfying your need for protein but you'll literally never get enough because you aren't eating protein means it has killed more people than rat poisoning or HIV or tuberculosis or most other things.