Nicotine by itself is pretty much harmless.
What matters is the affects small amounts of nicotine, repeatedly, have over time. Nicotine increases blood pressure and so it slowly destroys your cardiovascular system. Smoking is, of course, bad for your lungs.
The combination of lung damage and heart damage is deadly. Typically heart failure will require more oxygen in your blood to make up for the fact your heart doesn't work. But the lungs are bad at extracting oxygen when they have pulmonary damage.
I smoked for >20 years, quit for 10, then started again when dealing with grief after my partner passed away, and smoked for another 10. I get it, literally: high-blood pressure, etc. Although, when quitting, it was always the tar that really drove me crazy.
Another thing I've realized, is that smoking is a double-whammy vector towards heart disease: smoking promotes heart disease directly, but smoking also promotes gum disease, and gum disease promotes heart disease.