Killing Blow increases charge damage on a melee weapon by 25% per rank for a maximum of 150% at rank 5.
Note that this mod affects charge damage only (i.e. the melee attack that is performed when holding down the melee key for about a second or two, depending on melee weapon used).
Does not stack with Pressure Point since that mod affects regular (non-charged) attacks only.
Will not modify the extra damage dealt by elemental or armor-piercing damage mods (they all use the weapon's base charge damage).