In Batman vs Superman Dawn of Justice, Lex Luthor kills his assistant along with the others in the bombing at Capitol Hill. Why does he do that? She was loyal to him and did loads of his dirty work. So why would he kill her?
In my opinion, I think if she (Mercy Graves) wasn't killed, she would have known about the bomb. So Lex thought it was better for him to kill her than to risk her telling someone...
I found some other opinions (nothing official) that relate to mine like this one:
The price that Mercy pays for her sycophancy and bodyguarding Lex is her life
Or this one:
Mercy clearly walks over and looks down into the crate containing the kryptonite after the kgb beast delivers it to Lex after the batmobile/'do you bleed ' chase, and after she smiles at Lex gives her this very weird cold look. In the next scene the kgb beast kill the female 'witness' from the earlier trial by pushing her in front of the train, and the court scene is next! IT FEELS LIKE HE WAS TYING OFF LOOSE ENDS BY KILLING ANY ONE THAT KNEW ABOUT HIS PLANS OR KNEW TOO MUCH TO COMPROMISE HIM AND LINK HIM TO THE CONSPIRACY, and poor mercy just knew too much by looking in the crate, and in that moment lex realises she has to 'go' along with senator Finch and the wheelchair guy at the superman trial as he told her about the krytonite and wanting to import it and knew about the witness testimony being a lie. only a theory,this shows this Lex to not only be calculating and manipulative but would take you out if you went against him or could compromise him and his plans even if your considered 'close' to him. i like this lex more and more i watch this film and gives him room to grow in future films as his immaturity cost him this battle as he was so cocky and thought his rare kyptonian mutation added to zods body would allow him to control 'doomsday', but a genius learns fast and will be more and more like the comic super villain version the more we see of him.
Also we don't know if she is really dead, theories say that she could be an android...