They think they get more power from their acts of evil if they inform us of what they plan before they do it. To them it is akin to us consenting to it. Or maybe they just like to laugh at us. In any case, they usually feel compelled to tell us what they are going to do.
Example:
A witness to the Las Vegas shooting says concertgoers were warned that they were "all going to die tonight" before gunfire broke out, killing 50 and injuring more than 400 others.
They think they get more power from their acts of evil if they inform us of what they plan before they do it. To them it is akin to us consenting to it. Or maybe they just like to laugh at us. In any case, they usually feel compelled to tell us what they are going to do.
Example:
https://www.syracuse.com/us-news/2017/10/las_vegas_shooting_die_tonight_woman.html