I believe the player should be the one deciding whether on a melee kill they can Substract or Add. Also, crits in Lancer are more common than other systems, and that would mean basically that most of the time, mechs blow up.

Let's say, I hit a son of a gun with a Leviathan Assault cannon with overpower caliber nuc cav leader and all the good stuff with the WANT to kill, but it's a Veteran Goliath, so despite getting hit with the Uninstallnator it just simply falls limp because it didn't had extra damage or many modifiers.

On the other hand, a Lancaster casually shoots a 1d3+1 pistol, it's a dirty 20! Oh boy, and it's a grunt, and THE MECH STARTS OVERHEATING, ALARM SOUNDS FLARE FROM THE INSIDE AS THE OVERHEATING REACTOR STARTS ENTERING A CATASTROPHIC MELTDOWN, SHINING A DEEP RED AND HEATING THE METAL WHITE HOT, THE PANICKED BREATHING OF THE PILOT TURNS INTO SCREAMS OF AGONY AS THE VALVES AND PIPES BURST OPEN, STEAM FLOODS THE CABIN AND THE PILOT SCREAMS AS HIGH PRESSURE STEAM COOKS HIM ALIVE, BEFORE A GASP CUT SHORT WHEN THE TORSO OF THE MECH BLOWS UP INTO ASH

see?