Considering how he reacted after just watching his parents be murdered in front of him, you mean? On video, sure, but still in front of him when he'd had absolutely no reason to suspect it. I'm pretty sure in all the time Steve knew he could have come up with a situation where a) Tony had some lead up, b) Tony didn't have to watch it happen and c) where the unwilling weapon the murderers used, the face he's just watched murder his mother, wasn't standing right there.

All that aside, the fact that the family of victim might react violently is not an excuse to just never mention the fact that someone was murdered. Of course they're going to react violently. Their family was murdered. You still don't get to just keep it a secret because it's easier for you if you don't have to deal with that. Steve went to fucking war once upon a time. Cowardice is not an excuse to lie and cover up a murder in order to avoid potential violence.