Execute is an ability in the Warhammer 40,000: Space Marine single-player campaign. When enemies become stunned, they are marked with a glowing skull icon and Captain Titus can "Execute" them with a special slow-motion finishing move to regain health. The only other way to heal in the campaign is to use Fury Mode, making Execution an essential mechanic.
Certain weak enemies (Gretchin, Renegade Militia) can be Executed without stunning them. Larger enemies may require more damage or multiple stun moves before an Execution can be performed: trying to Execute one of these enemies without stunning them will cause an animation where they throw Titus away: Very tough enemies may require a short "struggle" QTE where the player must repeatedly press the execute button or the enemy will break free: this is skipped if Titus is in Fury mode or his melee weapon is a Thunder Hammer.
Captain Titus executes his opponents in short slow motion animations. Generally there are 1–2 unique Exection animations for each weapon–enemy pair (e.g., one for finishing a Slugga Boy with the Combat Knife, two for finishing a Slugga Boy with the Chainsword, etc.)