Are there any lethal melee weapons?

TL;DR: Invisible Inc has one lethal melee weapon, but you can't acquire it in an unmodded version.

There is an item hidden in itemdefs.lua. But first have a look at at scripts.zip/sim/abilities/melee.lua (186-192) for the melee ability (you can unzip the scripts.zip in the game's directory to get a look at the game logic and other interesting stuff):

local lethal = false


if simquery.getEquippedMelee( userUnit ):getTraits().lethalMelee then
    grapple = false
    lethal = true
end

So melee is by default non-lethal, unless the equipped weapon is lethal. itemdefs.lua now contains the following item:

item_baton = util.extend(melee_template)
{
    name = STRINGS.ITEMS.BATON,
    desc = STRINGS.ITEMS.BATON_TOOLTIP,
    flavor = STRINGS.ITEMS.BATON_FLAVOR,
    icon = "itemrigs/FloorProp_AmmoClip.png",
    profile_icon = "gui/icons/item_icons/items_icon_small/icon-item_tazer_small.png",
    profile_icon_100 = "gui/icons/item_icons/icon-item_tazer.png",      
    --profile_icon = "gui/items/icon-tazer-ftm.png",
    requirements = {  },
    traits = { damage = 1, melee = true, lethalMelee = true},
    value = 500,
    floorWeight = 1,
},  

All other items which extend melee_template don't set lethalMelee to true, so that one should be the only one. It's a monofilament baton, according to strings.lua:

MONOFILAMENT BATON

Melee strikes are lethal

Though it looks like a simple club, in reality it's a short whip, thanks to advances in molecular construction. The filament is nearly invisible to the eye, and the results are often as hard to look at.

But the item_baton doesn't come up in any other file. It doesn't get dropped by guards, nor will Monst3r ever sell it. So even though the game supports lethal melee weapons, you will never acquire one in the current version.