Lancer 4 E still able to reduce Max HP to below 0
For a more likely scenario, Avenger's True From gives a Max HP bonus. So it is possible to have HP reduced below the True Form bonus (and not meet the Kill condition). When True Form runs out, Max HP drops below 0.
https://github.com/dun1007/fateanother/blob/bf44cdbd721138cddec80eddfd6f4f0452cfbd75/game/dota_addons/fateanother/scripts/vscripts/diarmuid_ability.lua#L144-L146
The Kill Condition checks for less than, but not equal to. So on the occassion the enemy hero is at the exact HP where a multiple of E spears will kill, the Kill condition is not met.
Update: there is a consistent way to do this by using Lancer 4's E in conjunction with TA's Snatch Strike. If enough HP is lost by the strength reduction, it is possible to fall below 0 max HP (e.g. hero with 200 HP after Lancer 4 E gets 20 strength stolen).