>why Frank Grillo didn't just kill Cap once they
>secured the ship in the opening scene.

there really was no need to kill Cap at that point. they actually didn't go after Cap until the realized that he knew something after Fury was in his apartment. if Fury doesn't visit Cap and let him know that something's up, Cap would have just assumed that Fury was dirty like he was told and kept it moving. the plan was to get the 3 Helicarriers up in the air and they'd be able to take out anyone they wanted all at the same time.