The first 5 or 6 times I watched the show, I thought Yi was just in a chronic state of Gay Panic over his feelings for Diao, wrestling with his self-perception of heterosexuality.
But then I read the novel, you learn that Yi has been engaged to Diao since Yi was 16 or so, and absolutely loves every single hair on that boy's head.
The layers of grief and guilt between Yi and Diao are:
His arrogance and irresponsibility that led to Diao losing the memories of his childhood.
Being the reason Diao was sent to the other side of the world to grow up alone, away from family or friends, lonely.
The guilt he feels for killing Lele, for realizing a split second too late- as he pulled the trigger- that she wasn't attacking Diao and could be talked down (in the show, they sent her to a sanctuary, but in the novel he shot her in panic).
Again, Lele, her sister from the same litter is Lanlan, his father's puma, and every time Yi goes to see his father, Lanlan comes to greet him. It only reinforces his grief and guilt.
The situation between the Wongteerawits and the Chens, hating Diao's family for basically blackmailing the Chen's with the dog/puma/tiger incident.
Knowing the Wongteerawit's use Diao via the engagement- something Diao is aware of and also hates. Yi feels like cutting Diao loose might be what's best not for the Chen's (though, it would be good), but also for Diao himself. To free him from being used by his family.
So like, the man is carrying so much weight on his shoulders, it's amazing he can get out of bed in the morning. In the novel, he also has crippling insomnia and basically can only fall asleep if he's passing out from exhaustion.
Which I'm sure only feeds into his extreme temper.
