This is the 17144th most frequent Polish word.
zabiciem
The Polish word "zabiciem" means "killing" or "by killing" (instrumental case of "zabicie").
Here, 'zabiciem' is used as an instrumental case noun derived from 'zabicie,' signifying the act of killing.
Jego zabiciem zakończymy misję.
We will conclude the mission with his killing.
In this context, 'zabiciem' is again instrumental case, expressing means or manner connected with the verb 'zadowolił się' (he was satisfied).
Nie zadowolił się tylko jej zabiciem.
He was not satisfied with just her killing.
Although semantically odd in this example, 'zabiciem' was aimed to clarify usage but generally would not work grammatically here; proper context should reflect instrumental case role properly.