Henoch was a friendly and charismatic individual, who, despite his pleasant demeanor displayed psychopathic and sociopathic tendencies, who had no qualms about murdering and manipulating those around him, and abusing the abilities Spock's Vulcan body gave him.
In 2268, Henoch occupied Spock's body and decided that he would not give it up. He used Spock's mind meld abilities to confuse Nurse Chapel so that she would administer a worthless metabolic formula to Sargon occupying Captain Kirk's body, so that Sargon and Kirk would die. He then took control of the USS Enterprise. When the doctor tried to inject him with a (supposedly) deadly poison, Henoch ordered Nurse Chapel to inject the doctor. She seemed to follow this order, but as Spock's consciousness inhabited Chapel's mind, she was able to resist and inject Spock's body instead.
Henoch in Spock's body immediately perceived Sargon as being alive when he was injected. Henoch had presumed Sargon dead. Henoch then left Spock's body and tried to transfer to another body, but was stopped by Sargon, so his consciousness died. The injection had not really been deadly, as it was used as a ploy to drive Henoch from Spock's body. (TOS: "Return to Tomorrow")