The bullet from the gun causes death
Guns do kill people; more specifically, the bullets released from guns kill people. It is scientifically inaccurate to say that guns don’t kill people because they carry the agents that do the killing within them. The agents are bullets. The kinetic energy from the bullet, when transmitted into another person’s body, damages organs, break bones, and destroys cells and tissue. If a bullet enters a person's body and hits critical organs, that person is going to die. The bullet will have killed them. The guns themselves are the actual instruments of death. For instance, just as the HIV virus is the agent that causes AIDs, the bullets that are fired from guns are the agents that cause large-scale death. Guns do kill people because they harbor the bullets that do.
Guns do not kill people. Even if a bullet is the one that directly causes death, this cannot happen without the action of a human being. Death cannot happen without a person who pulls the trigger and releases those bullets. People with no moral compass, with mental illness, or with no sort of sanity allow such agents of harm to kill innocent people. It is imperative that the true problem is addressed, which is the people who are doing the actual killing. A gun does not become a dangerous weapon unless it is in the hands of a dangerous person.