Neyra Drake stealthily approached the target of her mission - Alexeagle, but she was prevented from completing the task by VesperLindt, who arrived on mechanical wings. Several bullets hit the assassin and took her life. Alexeagle wanted to thank his savior, but the blade that Neyra Drake managed to plunge into VesperLindt's back in her last breath killed him, depriving him of the opportunity to do so.
Neyra is tactical. Stealthy infiltration. Alex's suspicion yesterday. Vesper tried to protect Alex - an Angel. Neyra used her special ability.
Tactical is a special ability. Last mission: takes the life of her killer.

The last mercenary from the Syndicate decided to use a prototype and injected a peculiar syringe into his neck. A couple of minutes later, he felt significant changes in his body, but more importantly, his strength and abilities increased many times over. His brain, on the other hand, became very unstable, and, on the orders of Ashurbanipal, he went to kill Mona. He easily took the life of a person who was busy repairing equipment.

Mona is an engineer. What kind of syringe did the killer inject himself with? If it was Walker's ultimate ability, he wouldn't be able to kill. But here he killed.

Based on the clues http://ru.wikipedia.org/wiki/Библиотека_Ашшурбанипала
it seems to be the most likely for now.

Among the peaceful ones, I see Ment, Drakosha, and someone from Amco/Alexigle.
I'm playing for the city; it seems that four city dwellers are still alive.
Arandon/in the game/ is suspicious, as is Sesam/clue and game/, Cassi, and the mold - I don't understand their game.

I would execute Sesam.

Added 1 minute ago
I also forgot about Nebo; he doesn't seem to be for the city either.