I read somewhere that a primary cancer seldom kills ; most of the time the metastasic-induced does. Why ? There should be far more "bad" cells in the primary site, doesn’t it ?

The most anatomical origins for cancers (skin, breast and prostate) are not vital organs, so a cancer in one of those places won't kill you if it stays in place. The danger is if it metastasizes to a vital organ (most commonly the lungs or liver) and interferes with its function. If a cancer starts in a vital organ, it's more likely to kill you through its effects on that organ, not by spreading elsewhere.