I think the matthias hate is so forced, people forget that all the crows have their flaws, and they're all morally grey. And I think that people assume 19 is a very big age, that after 18 a switch just clicks and every mistake after that is unforgivable, when that is simply not the case. Ofc, this is NOT to say that his hatred was justified, not at all.
But let's say, in these times, teenagers have so much access to different forms of media, people from different cultures, etc.— it's privilege in a way to know how the world works and look beyond prejudice and stereotypes, and to form opinions of your own.
Matthias, on the other hand, lost his parents when he was young, and was quite literally raised by a dictator. And YET, he believed that the Grisha received a fair trial and a chance to explain themselves. He did not know how deep corruption ran, and how it would come to intertwine with his faith. He was groomed into submission marketed as 'duty', and he still managed to unlearn it, and in doing so experienced a love so profound and soul-consuming, that in his last moments all knowledge was a distant memory, save for the love he felt for Nina.
