Because malignant narcissists are seething with inner rage, and filled with envy, they cannot be happy for someone else. In their minds, your success or good fortune steals something away from them. Perhaps you're in the spotlight. They can't handle it, because they need to always be the center of attention. Perhaps you've won an award. Someone with such a severe character flaw believes it should have gone to them, even if they've done nothing even to earn it. and even if they were not in the race to receive this honor. (With a narcissist, whatever you have, they want, even if this possession is of no value to them.)
Because a morally disordered person's mind is so twisted, and because these shallow individuals are so self-absorbed, they cannot truly be happy about another's accomplishments or blessings. Someone who is extremely narcissistic, or malignant, will even take the added steps of trying to take this accomplishment or praise away from you, or try to clip your wings, so you land with a big public thud. This, she believes, will raise her own standing.
Do not mourn if you suddenly lose a "friend" because something good has happened in your life. This person was never a friend in the first place. There is such a thing as foul-weathered friends, as opposed to fair-weathered friends. The foul-weathered folks are at your side while things in your life are falling apart. However, they get very upset when things take a turn for the better. Avoid these types. They will only cause you misery.