Yes we have them, any weapon designed to operate without direct human control is in fact a killer robot, or to use a term borrowed from science-fiction, a Berserker.

Mines are a classic case, and they've been around a long time. Essentially, they are "let loose" on an enemy, and they kill automatically, without any further intervention or control.
They can be extremely low-tech, or programmable and even "smart", like some naval mines, capable of primitive decision-making.

Missiles and torpedoes also exist that are not only capable of being aimed and fired at a target, but that can simply be let loose in the general vicinity of one and instructed to search for it, while ignoring targets it identifies as friendly.

Of course, most robots are incapable of distinguishing between friend and foe, and pose a threat to both sides, or to neutrals, sometimes to non-combatants in the future, as did the Berserkers in Fred Saberhagen's novels. Even some artillery is now automatic, such as the Phalanx system and many radar-controlled anti-aircraft systems.

I submit this is one bridge that has already been crossed, in fact, hundreds of years ago in some cases. The idea of a robot tank, or aircraft, operating autonomously behind enemy lines looking for a target is fundamentally no different from, say, an Exocet seeking out the enemy fleet.

In a sense, ballistic missile systems are also robots, even though many humans may be involved in their operation. During the Cold War, many retaliatory weapon systems were designed so that the enemy would know that no matter how well they damaged our command and control system, the missiles would get launched anyway. All those men in silos, bombers and nuclear submarines, looking up codes, turning keys, pushing buttons are drilled continuously to simply go through the motions once the command is given. They might as well be robots, once the order is given, nothing can stop them, indeed, they are indoctrinated to ignore a command to stand down, which might be an enemy trick.

It's too late to worry about it now.