Thomas Wayne was killed when Bruce was 8 years old.
There have been various instances where Thomas Wayne becomes Batman. The most notable example is in the Flashpoint story arc, where Bruce Wayne is killed instead of Thomas and Martha. Bruce's death prompts Thomas to become Batman whereas Martha Wayne is driven to insanity and becomes the Joker. Unlike Bruce Wayne, Thomas Wayne does kill people as Batman and is willing to use guns. The other instance of Thomas Wayne becoming Batman is in the Earth 2 run, where Bruce sacrifices himself to save earth from Darkseid. Thomas takes up the Batman mantle to continue his son's work. Like in Flashpoint, Thomas Wayne does kill people while he's Batman and he even kills the Joker. Both versions of Thomas Wayne's Batman are featured as skins in the Batman Arkham games and the Flashpoint Batman is a skin in Injustice Gods Among Us.