Klaus Voorhees was a HYDRA scientist inside SHIELD before its fragmentation. He studied snake venom's and how to apply them to advanced bio weaponry. When SHIELD broke apart Klaus murdered his superior with his experimental venom. HYDRA lost the facility Klaus was stationed at, so in order to avoid being incarcerated he injected himself with his toxin to give himself the appearance of death. When he injected the antidote into his bloodstream he was mutated and become King Cobra head of the Serpent Squad.
Elasticity: King Cobra possesses certain unusual physical powers derived from his mutagenically altered physiology which among other things help him simulate the movement of a snake. All of the bones in his body, including his skull, are malleable and his muscle tissue is exceedingly resilient, making his body very flexible and pliant. It is nearly impossible for him to break a bone or tear a muscle. Hence, the Cobra can survive impacts that would kill or inflict major injury upon a normal human being with little or no injury to himself.