User blog comment:BlackWizardZeref/Fairy Tail Chapter 384: Attack of the Celestials Review/@comment-24572618-20140517024742/@comment-7418318-20140518085517

The first thing Jackal did after being revived was getting pissed at Lamy. He very clearly dislikes her.

And before he killed her she was interfering with his "fun" and despite Jackal being the one who did all the work, while Lamy just got kicked around by Virgo, Lamy still behaved as if she deserved the promised promotion. And she never shuts up.

Plenty of reason to kill her, if you are a Demon with a short fuse.