The Sims 3 on Xbox 360 (and all platforms) is a life simulation focused on creativity, storytelling, and player agency—not violence. While the game allows Sims to die through gameplay mechanics, there is no "kill button" or murder interaction. Deaths occur as consequences of player choices or neglect.
Dead Sims become playable ghosts (with unique interactions) and leave tombstones/urns. Other Sims mourn, get "Mourning" moodlets.
If you want challenge: Try legacy challenges, rags-to-riches, or "Black Widow" (marry → inherit → repeat) — all within the game's systems.