Emiel being harsh and cruel on training is actually not weird to me. Especially from his experience as a guardian of Estelle. He said that he will sooner die than let Estelle be hurt, but it turned out different.
Probably the helplessness that he felt when Estelle was kidnapped made him develop this side of his.
Is it bad that he is harsh and cruel? Well, if it's during a training of a regular army, yeah it's kinda bad. Lotsa people will just quit.
If it's during training of special unit (of some sorts) which are the knights (not just a regular troop but a true knight) then being determined to succeed, to always be better will give better results.
That's how the modern special forced train, they force people to quit and when they don't, they are fit to be the best.
The other matter is his loyalty. Is he unloyal to Estelle by hiding his cold side? Or is he cold and harsh on others for her sake?
Or is he just simple a little sick in the head and he just can't help it anymore, not sure about it xD