I think I've read something in the structure of "he is [adj] [prep] humanly temper", but I can't remember the exact words for someone who should be just but instead let the temper got the better of them. Kind of like Greek gods who get carried away by little things just like a real life human being. Is there really some phrase like this?
My humble suggestion is
He lets emotions get the better of him.
He fails to walk the line.
One of the definitions for "walk the line", per Wiktionary, is
(idiomatic) To behave in an authorized or socially accepted manner, especially as prescribed by law or morality; to exercise self-control.
"Walk the line" and "toe the line" are idioms more appropriate when describing a person in a subordinate position, or trying to conform to other people's wishes and/or expectations.
"Get the better of" could be more appropriate if your goal is to describe, say, a high judge in a country with powerful and independent judiciary. He wants to be impartial out of considerations of honor etc., but sometimes emotions get the better of him. After all, he is a human being.
He fails to keep his emotions in check.
He is driven by human passions and beset with human frailties.
In English, we don't have a phrase for people being too emotional for their position. The closest I can think of is something like "they're very sad for being a clown," or something of that sort where the person's emotions do not seem to match what you would expect from their work.
However, for someone who gets angry easily you could say they are "hot tempered" or "hot headed." If they are just very emotional, you can call them "overemotional," but this is regarded as an insult.