Weapons that automatically kill people are prevalent. It may seem trite, but that's exactly what most mines do. Some anti-ship mines are smart enough to distinguish friend from foe using acoustic classification and would be programmed to only kill enemy ships and ignore friendly ships.

Whether or not that constitutes "AI" (pointless discussion since nobody can decide what "AI" truly means even in conversations about state-of-the-art LLMs), it's a weapon autonomously monitoring its environment, classifying targets and killing them without a human in the loop.