For the ruthlessly brutal galactic pirates, they had no mercy at all, but that was still over five hundred lives. To have them kill so many people all at once was something they found somewhat difficult to do.
Another way was to immediately return, taking these warships back with them. However, if they were to take them back to the Mother Planet, there would be problems too. First of all, those damaged warships might not withstand the traversal through a wormhole. Moreover, it would delay the execution of their mission.
The battle earlier wasn't quiet and may have already alerted other pirates. If they were to return now, their identities as members of the Thirty-Three Skywing pirate group would probably be exposed. They would have to assume a new identity next time. This was against their original intention.