How do I kill Ambassador Brenner?

According to the wiki you should 'just' be able to kill her, but it's important to disable 'safe mode'.

Talk to Mythany Keen in the Embassy Hall in Ysa. She will ask you to meet her in the worship circle near the city gates after dark.

When you meet Ambassador Keen, she tells you the Varani are planning to attack Ysa, so you should kill Ambassador Brenner, steal her Daggers and get rid of them.

Ambassador Brenner takes a walk at the Memory Walk. Just follow the quest marker and kill her. Take her Daggers and AFTER talking to her, either salvage them at a Forge or sell them. Again the quest marker will guide you. Salvaging them PRIOR to talking to her after the murder, will cause a quest failure. Return to Ambassador Keen and take your reward.

You can also tell Ambassador Brenner about the plot against her. She will ask you to kill Ambassador Keen instead.

Wait until night and assassinate her near the shrines. You will want to push up on the d-pad and sneak attack the ambassador. Take the daggers and take them to the location requested by Ambassador Brenner.

If you for some reason cant follow the steps mentioned on the wiki i would guess you've ran into a bug.

Your best bet would be to submit a bug ticket at the EA helpdesk.

I could not enter aggressive mode. So I pick pocketed her on memory lane at night when she WOULD notice, the pickable items don't include the daggers, but once she yells, " thief", you can kill her, and the daggers will be on her body.