Who is it that killed Zephyr's crew and then became a warlord?
The Warlord who killed Zephyr's crew is not named even in the current chapter of the manga.

Nevertheless, it was stated that the pirate who killed Zephyr's crew became a warlord during the timeskip. Therefore, there are only three possibilities: Trafalgar Law, Buggy and the currently unnamed warlord. However, it it also not yet revealed whether the unnamed warlord is a devil fruit user.