How about if an enemy-held system had way too many targets to use the Behemoth effectively as a planet-killer? For example, if the Kilrathi had a system with two or three heavily inhabited planets, military bases on more than a dozen moons, and dozens of spacebourne stations spread through the system. It would probably be simpler just to make the system's sun go supernova and destroy everything at once.
As for messing with gravity, after a supernova, 98% of the matter is still there, only it has been scattered about. Likely, any jump points close to the star would get messed up since there is now a nebula with a pulsar in the middle instead of the original star, but ones farther away might stay intact.