Might as well just read something else

point
1-2 • It's obvious because this was the background of the MC, and as we all know that the MC need to be OP.

3 • He did the right thing, he was framed and he have no strong evidence to support his innocent. Thats why he run, He might be strong and might kill most of the guards and the king, but after that what would happen?
he will just show himself to other people that he is indeed the one who do the evil thing.
Exemple : You have the power to kill a President and his bodyguard, but after that can you fight all the Netizen? which one will those people trust? the STRANGER or their own leader the PRESIDENT?

4• At that time he just found out 1 Crystal, he never know the other crystal. Go re-read again if you think i mistaken.

5• Well that's the human nature, That's why in real world has "Crazy" people, When they can't handle some psychological pressure or suffering under hard circumstances they will become "Crazy". In this case the King and Princess become twisted evil.

6• It is the same as point •3 ,if in some kingdom have 2 factions they can't do something so obvious, and in this manga set-up they were using Royal-Noble-Knight system which has their own etiquette code, even though they wan to do something they just couldn't without strong proof to justified their own actions to Audience/Netizen/Outsider/People.
This is what they call "POLITICS".

7 - 8 • Because it was the last wishes from the dead knight that teach him before. He said that MC need to clear that Dungeon and grow stronger if he wants to survive in that world. And so then he also give MC the "Katana" since it was Japan weapon and this manga is from Japan.

I'm falling on an illustration where Amelia wears a ring on the wedding finger, Akira plans to marry her in the novel ?