There is mercury inside a thermometer, wouldn't it kill a person if it popped open?

It will be rare for the contact with mercury to cause death if the person reacts quickly and takes mercury poisoning treatments, but the effects of mercury poisoning appear slowly so if a person comes in contact with mercury it will be hard to say, but the lack in treatment may cause death

Not all thermometers contain mercury, but if you came into contact with it it is unlikely that you would die if you were treated, however it can have some pretty adverse effects such as peripheral neuropathy, showing as itching, burning, pain, or even a sensation that feels like small insects crawling on or under the skin (formication); skin discolouration (pink cheeks, fingertips and toes); swelling, and peeling skin). Other effects include profuse sweating, high blood pressure, a faster than normal heart rate, more saliva. Affected children might show red cheeks, nose and lips, loss of hair, teeth, and nails, rashes, muscle weakness, increased sensitivity to light, kidney dysfunction neuropsychiatric symptoms such as memory loss, or insomnia.

Extra information: this happened to me once, it was not pleasant.