De Niro has a dark side. One of the Q threads of late dove into his past focusing on some child trafficking/prostitution rumors in France in 1998. Also, there was some highlighting of the dark subject matter in the movie Taxi Driver.
There is something definitely "not right" going on with him, and I truly think it's his fear that Trump "knows", and Trump may very well expose it all. For all I know, De Niro and Trump could have been at an event in the same room and Trump heard things. Heck, it could have been even more direct: De Niro might have been popping-off right to Trump's face about "inappropriate subject matter" in order to impress, be cool and "down with it" in an attempt to bring Trump on into the "game." In the circle that Trump traveled in...business, global business, entertainment, NYC society...no telling the stuff Trump has heard over 50 years. And all the time Trump is smiling, nodding his head, offering a few you don't says? at the correct time to be cordial and keep the conversation going mentally cataloging tidbits along the way. It can't be emphasized too strongly that Trump, the non-drinker, has his antenna array reading 5:5 at these gatherings.
I cannot think of any other reason why the mere mention of (even thinking of) mentioning Trump's name sets De Niro off with viciousness. Zero to sixty at the flip of a switch. The only other derp I can think of that reacts in that manner is Jim Carrey. Both of them do instantaneous, vertical ramp-ups of anger toward Trump. Father Merrin tossing holy water on Regan?
We will eventually find out what's been going on. It can't come soon enough for me though.
Seems he was a close friend and cohort of Roman Polanski