So much hate on this knight dude. Sure, he's a little edgy, but he's not wrong.
And considering that Tina was original fated to die before the time rewind, it fits the tone of the manga better than "oh, you restrained a couple knights after having all possible prep-time in laboratory conditions, let's train you for real war".
Also, even if Eric comes across this situation, what do you think he'll actually do? Refuse to join the knights because knight dude held a sword to his sister? Nope. If anything, he'll be mad that Tina wants to follow him somewhere dangerous. He knows he's lacking right now, and the knights is the first place for him to grow. Maybe he'll change his mind and decide if Tina can get stronger, he'll worry less, but it would be completely out of character for him to rage at knight dude for teaching her the reality of battle.