Keyword Ability. Any damage a Lethal unit deals to a minion is enough to kill it. (Remember that zero damage isn't damage at all, so it will need to do one or more damage.)
For example, can kill with a single strike for 1 damage. If the Amazon Warriors had their nearby, then the Pit Vipers wouldn't actually kill the Amazon Warriors, because they'd do zero damage, which isn't actually damage at all.
A spellcaster with the Lethal ability still casts Magic spells normally; the Magic spells themselves deal the damage. For example, a spellcaster with Lethal casts , but the damage from is not Lethal; Firebolts is dealing the damage, not the spellcaster. On the other hand, the damage from is Lethal, because the spell itself says it's lethal.