DNA: What is saccharin and artificial sweetener?

You might remember that last month on March 24, there was a news from Punjab... in which a girl had died after eating cake on her birthday in Patiala district of Punjab... Then questions were raised that how can eating cake cause harm to anyone? After which 4 samples were taken from the bakery from where the cake was purchased. The report revealed that out of the 4 cakes whose samples were taken, synthetic sweetener was added in 2 cakes. What is artificial sweetener and why is it called sweet poison?