9. KGBeast - Batman#57
Anatoly Knyazev, otherwise known as the Beast (the moniker KGBeast was given to him by the CIA) is a Soviet-era assassin who was hired to kill various high-ranking American officials in order to break down US/USSR relations at a time when the two nations were working towards fostering peace.
Batman gets wind of this plot and confronts the rogue agent, but fails to stop him due to Knyazev's then-superior skill. Their second encounter saw the Caped Crusader fare better and lock his foe in an underground room, essentially leaving him to die.
This act would later be retconned but it would not stop Bruce from showing no mercy towards the assassin. in 2016's Batman #57, the hero grievously wounds and abandons KGBeast in the frozen tundra, essentially sealing his fate.
While this event was once again changed in later stories, it is interesting to note that this relatively obscure villain garnered so much wrath and mercilessness from the Dark Knight on multiple occasions.