As the sequence begins, keep heading onwards until an opera employee blocks your path. Go inside the row on the left and sit down on the only free place.
During the conversation you will have to use the Eagle Vision and look at the man sitting in the loge on the right.
After regaining control over your character, head left. There you will find a ladder using which you can get onto the wall of the opera hall.
You need to sneak between the balconies to the right. If you find it impossible to move further into that direction, try climbing one level higher and follow the green point. Eventually you will reach an empty loge.
To the right there's a door which you have to open with a lockpick. Press the interaction button to begin an easy mini-game.
Start off by moving the left and afterwards the right analog stick until both lockpicks are in proper positions. With the lockpicks adjusted properly, keep holding the sticks and start pressing the free run button. That way you will open the lock.
On the other side of the door, jump onto the element of the scenography and head further to the left.
That way you will reach a ladder which you have to climb back up.
From there, jump onto the wooden moon and head to the left edge of the scene.
Behind the door you will find your victim.
Kill the man with the attack button and afterwards move through the crowd to the opera exit, pointed by the green marker. The mission will end as you enter your carriage.