Charles Cullen does not use any kind of knife or guns to kill people but instead he uses Medical instruments like drugs that the patients body will react badly to. Here are some examples of the medical instruments he used:
Insulin is a hormone that lowers the glucose of the blood. Charles uses this stuff to kill his victims by putting them in their IV Bags.
A poisonous compound present in the foxglove and other plants. It is a steroid glycoside and is used in small doses as a cardiac stimulant.
Epinephrine (also called adrenaline) is the principal blood-pressure-raising hormone secreted by the adrenal medulla, is prepared from adrenal extracts or made synthetically, and is used medicinally especially as a heart stimulant, as a vasoconstrictor (as to treat open-angle glaucoma and life-threatening allergic reactions and to prolong the effects of local anesthetics).