After Roan arranged a guardian meeting, Jeff went to Amsterdam. Michael and Owen attacked him in the alley. They were able to snap his neck together. They took a blood sample from him, his black box and a note with the location of the meeting.
As a guardian he was a highly skilled operator. He was a skilled fighter and was able to use acrobatics and powerfull kicks to fight both Owen and Michael before he got killed. Because of his status as a guardian, his skilled were further augmented by the Regimen, which enabled him to almost take down Michael and Owen single-handedly.