visiting Talos IV is the only death penalty
Well, let's see.
Visiting Talo Four could be the only death penalty within Starfleet's justice system, while in Federation member's civilian courts the death penalty is the preferred consequence for murder/homicide. Especially if you're killing people by the hundreds.
M5 is debatable a self aware sapient being, but it isn't a Starfleet officer or crewman. M5 is a "civilian."