One of the space invaders alien ships, followed by many more of them as they continued their grand strategy of moving horizontally across the screen, then advancing one piece, then moving back horizontally across the screen, repeat.
The next wasn't until a few years later, being the ghosts in Pacman (i.e. when you eat the corner dot and are able to turn the tables and chase/kill them for a few seconds).
In the years between those two most games (that I played) didn't involve killing - usually you'd be jumping over obstacles and avoiding things with no means of attacking back (Donkey Kong, Pitfall etc).
If we're talking games that I really got into and played to death, the first one that involved killing was Wizardry 1, which I remember playing in 1984.