The kill script is disabled until another script activates it, the kill brick has CanCollide off and the player is basically inside it when seated, but when it activates it does no damage to the seated player.
I've tried to use this code but when it activates it just won't work
function onTouched(part)
local h = part.Parent:findFirstChild("Humanoid")
if h~=nil then
h.Health = 0
end
end
script.Parent.Touched:connect(onTouched)
Please help.

4 comments
If I'm not wrong, the touched event requires CanCollide to be on in order to activate. Did you made the kill brick collidable?
I've tried on hit too and yeah, its CanCollide off
Turn on CanCollide
You could check the Humanoid position Humanoid.Seated and use Humanoid.SeatPart to get the seat and trigger damage as well