Dishonored is a confused game. I don't think it figured out whether it wants to be a stealth game, or a violentkillkill game.
The main action button is the one that stabbed your knife straight ahead, and the secondary action be whatever you had equipped.
And because by default they're mapped reversed (left-click is the action with your right hand, stabbing faces, and right click is secondary action with equipped item), I was constantly going up to people, trying to use the heart on them to read their minds, and consequently stabbing them in the face.