Are we right to be worried about my brother's ex girlfriend messages?

He broke up with her several months ago for jealousy and trust issues. Once in while, he receives insulting messages on his fb from different accounts.
Then 4 days ago, he received a missed call from her. This was what the voicemail said ''I swear you're gonna regret breaking up. I love you so much. You're mine. If I can't have you, then no one can. Just remember that''.

Guys, how would you react if an ex girlfriend left that message? Are we right to be worried?
Are we right to be worried about my brother's ex girlfriend messages?
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