Jasper Sitwell was a HYDRA infiltrator who posed as a high-ranking S.H.I.E.L.D. agent. Sitwell had been indoctrinated into the ideology of HYDRA during his education with the HYDRA Preparatory Academy, for whom he took on a role as a double agent deep within S.H.I.E.L.D.'s ranks.

Who killed Agent Sitwell?

Sitwell, like most of the S.H.I.E.L.D. leadership at the time, is seemingly killed by a self-aware, renegade "Deltan" variety of S.H.I.E.L.D.'s artificial human "Life Model Decoys", and replaced by one such LMD, which was then installed as executive director.

Is Agent Sitwell good or bad?

Jasper Sitwell was a supporting antagonist in the Marvel Cinematic Universe, appearing as a hidden antagonist in Thor and The Avengers, a major antagonist in Captain America: The Winter Soldier, and a minor villain in Avengers: Endgame.