How many people does the monster kill?

The monster directly kills three people in Frankenstein: William Frankenstein, Henry Clerval, and Elizabeth Lavenza.

William is the first direct victim. The monster encounters the boy near Geneva and hopes he might become a companion because he is young, then becomes enraged when William reveals that he is a Frankenstein. The monster strangles him. Afterward, the monster frames Justine Moritz by planting William’s portrait in her pocket, which leads to her false conviction and execution.

The monster later kills Henry Clerval after Victor destroys the unfinished female creature he promised to make for the monster. Victor is accused of the murder when Clerval’s body is found with marks on his neck, but the killing is part of the monster’s revenge against Victor.

The third direct killing is Elizabeth Lavenza, Victor’s wife. The monster had warned Victor that he would be present on Victor’s wedding night, and Victor mistakenly thinks he himself is the target. Instead, the monster murders Elizabeth, completing his effort to make Victor suffer the same isolation he has endured.

Additonally, the monster is indirectly reponsible for a few characters' deaths. Its actions lead to Justine’s execution, Alphonse Frankenstein’s death from grief after Elizabeth’s murder, and Victor’s death during his obsessive pursuit of revenge. So, by the end of the novel, the monster has committed three direct murders, and its revenge has caused several additional deaths.