Donald "Red" Grant in general. There's rarely been a more evil, stone-cold, implacable psychopath in the entire Bond series. Every time he shows up, someone dies.
A special mention has to go to his murder of the Bulgarian agent in Hagia Sofia. Grant doesn't need gadgets or weapons, he can kill a person with just a couple of blows.
The very opening scene. It looks like Grant has killed James Bond by brutally strangling him, with the victim completely helpless... until it's revealed it was a human practice target wearing a mask imitating Bond's face.
For that matter, SPECTRE henchmen using humans as practice targets.
The scene where Blofeld (off screen) describes how Asian Fighting Fish pick off their tired opponents, and how he compares them to how SPECTRE operates.
For that matter, Rosa Klebb's death near the end of the movie. After getting shot, she gives off a death wail, before slowly crumpling to the ground for like a minute and a half, mouth still open in pain. And all of this is on-screen.