Fonda hates Trump with malicious contempt. I imagine he posted this tweet with joyous satisfaction and indignant justification. I hate Trump too. I hope I am never so lost in that hate that I would post a tweet like this.
Most actors are too stupid to be commenting on politics, and their livelihood depends their popularity. Hating on Trump will never make them more popular. It will alienate a large segment of the population as will taking a stand on almost any issue.
I have to wonder what kind of bubble Fonda lives in that he thought this was acceptable discourse.