
Time was, videogames let you just kill whoever. Plucky sidekick, love interest, authoritarian father figure, plot-critical NPC—all these and more were fodder to a sufficiently trigger-happy player.
The best games to ever do it were, I reckon, Fallout: New Vegas and The Elder Scrolls 3: Morrowind. The former was designed to adjust the plot whenever you summarily executed a critical NPC, while the latter just flashed up a card whenever you merked someone vital to the main quest, suggesting that you might want to not do that. Maybe. If you like.