This sounds very like Roadmarks by Roger Zelazny. It has a left behind robot that is now a potter which is tasked with killing the main character. As in your description the robot was left as it was broken.
The novel postulates a road that travels through time, with a nexus placed every few years where a handful of specially gifted people are able to get on and off. While there is a plot involving a series of assassination attempts on the protagonist, the novel's main strengths lie in the unique nature of the setting, character development, structure and the short vignettes on each of the would-be assassins.
The robot is living under the name of Mondamay, but is indeed engineered for killing.
"I know that, here in your branch, a hypothesis which intelligent men
find laughable in my own branch happens to be quite true: namely, that the
Earth was visited long ago by creatures from another civilization, creatures
who left various artifacts behind them. I know you are such an artifact. Is it
"It is correct."
"I know, further, that you are a fantastically sophisticated death
machine. You were designed to destroy anything from a single virus to an
entire planet. Is that not correct?"
"It is so."
"You said that I am not the first such traveler you have encountered. I
know this, for I know the other man's identity. His name is Red Dorakeen, and
soon he will be seeking you on this branch. I have need of a very large sum of
money, and I will be paid it for killing him. I always prefer working through
intermediaries, however--human or mechanical--in matters of violence. You are
to be my agent in this matter."
And, indeed, the weapons are not functional:
"Halt! Right there!" John spun and raised one hand, his eyes flashing.
"If this is a trick, it won't work. And if it is not, I welcome the
opportunity to kill that bird with this same stone. Mondamay! Burn Red
Dorakeen down to a cinder!"
Mondamay raised his right arm, extruding a tube which he pointed at Red.
Lights flashed in the vicinity of his shoulder. There came a crackling sound.
A tiny wisp of smoke curled upward from out of the tube.
"Shorted again," he declared.
"What do you mean 'again'?" John said.
"It's been that way for thousands of years."
The only real difference is that Mondamay was already friends with Red before John showed up.