In AD&D, lighting bolts would bounce off the walls (back at the caster in 1E, using geometry in 2E) unless the wall was a conductor. Like most spells in 5E, everything has been simplified, so this is entirely a DM call. I would suggest if you do have the lightning bounce, that you only deal regular damage to each creature in the area, even if a creature would be struck more than once (perhaps granting disadvantage instead).If you are playing on a grid, and using the grid rules, then you couldn't aim the lightning down to miss the maw demon unless you were 5' up in the air (so that the line could follow the z axis). Arguably, you could drop prone, and cast it upwards.
If you are doing theater of the mind, I could see the argument made, but would grant half damage on a failed save, and no damage on a success for the paladin (and possibly advantage as well).
As someone else has pointed out, in this example the cultist could have simply moved to a diagonal space, avoiding the argument entirely.
ncG1vNJzZmivp6x7prrWqKmlnF6kv6h706GpnpmUqHy4tMSnZJplnJ60qcDNoqWgZZKkubV50qmcpaRdorK1edOhnGaenKS8s3qVbmpucWZk