現場に彼が現れてさ
たぶん殺さなくていいモンをさ
My understanding of the sentence is akin to "He appeared at the place. Perhaps, it won't be necessary to kill him"
This is said by a character overseeing the events from afar about the appearance of a certain person.
However, what does をさ indicate?
After that the scene ends and it moves to some other conversation.