Overkill occurs on both sides. It's also very subjective. A fighter innocently doing missions might not deserve a battlecruiser blowing it to pieces in two volleys, but fighters/bombers threatening said battlecruiser and calling in reinforcements... Is it overkill to kill them? Before -won't name- various Corsair gunboats, cruisers and battleships are called in to waste the poor Core captain?
Obviously, the same thing can occur to corsair capital ships. Is it overkill for them to fire at bounty hunter bombers eager to collect a bounty, before the initial volley of supernovas into their shields?
Funnily enough, there are no restrictions nor complaints against overkill from fighters and bombers...