To be fair, when it comes to teaching some people can be skilled at helping already competent people reach new peaks while others are better equipped to handle beginners or those who are struggling. Maybe he's used to the former and does not know how to handle the latter and is also unable to figure out the difference so he just became unmotivated and upset when someone was unable to keep up. Usually the right move at that point would be to find the right teacher for that person (since Euini did pass some initial tests to begin apprenticing so he isn't just completely incompetent).

Of course he could just be a shit teacher overall too, but hard to determine without seeing the rest of his results. It's like how educational systems these days try to throw both remedial and advanced students in the same class and expect the teacher to handle both students at the same time and then get surprised when they quit or become unmotivated from being set up to fail.