It's easy to imagine specific scenarios, especially when generalizing from fictional evidence. In fact we don't have evidence sufficient to even raise any scenario as concrete as yours to the level of awareness. ... I could as easily reply that AI that wanted to kill fleeing humans could do so by powerful enough directed lasers, which will overtake any STL ship. But this is a contrived scenario. There really is no reason to discuss it specifically.

A summary of your points is that: while conceivable, there's no reason to think it's at all likely. Ok. How about, "Because it's fun to think about?"

Actually, lasers might not be practical against maneuverable targets because of the diffraction limit and the lightspeed limit. In order to focus a laser at very great distances, one would need very large lenses. (Perhaps planet sized, depending on distance and frequency.) Targets could respond by moving out of the beam, and the lightspeed limit would preclude immediate retargeting. Compensating for this by making the beam wider would be very expensive.