So, something about Turtle Temper has wormed into my brain like the villain song in a Disney musical. The fandom correctly identifies the way Leo and Raph’s relationship mirrors Yoshi and Saki’s relationship, however, while we the audience map Leo to Yoshi and Raph to Saki, Splinter does it backwards.
When he talks to Raph about his temper, he says Raph’s anger reminds him of himself. Splinter sees an echo of the rivalry he had with Saki and projects it onto Leo and Raph, but with Leo as Saki and Raph as himself. When Splinter sees Raph he sees his own anger, frustration, and failures, and he thinks “If only I had controlled my anger, Saki would not have killed Tang Shen” when we the audience know that’s utter bullshit.
It’s very interesting to have Splinter think this way, and it logically follows through that someone with survivor’s guilt would find some way to blame himself for someone else’s horrible actions. And we can see how it’s negatively affected his sons.
Splinter likely started to praise not showing emotion and punishing emotional outbursts. That can look like good parenting; it can look like he’s teaching his sons to control themselves, but it produced one son terrified of expressing himself and another who feels he can only express emotions in aggressive outbursts. Leo gets rewarded for suppressing himself by being given more attention from Splinter, while Raph gets punished for having emotional outbursts by his feelings and opinions being ignored. Splinter tells him to ignore his feelings, to not let them affect him, but that ignores what causes those feelings.
Yes, it’s Raph’s responsibility to control his behavior and not let his anger put his family in danger, but Leo ignores Raph’s input on missions and Splinter puts the blame for most, if not all, of their disagreements on Raph. Raph is the one who’s uncooperative, he’s the one disobeying orders, he’s the one starting the fights with Leo, even when Leo’s wrong. It’s not surprising that if no one listens to Raph until he’s in their face or yelling at them, that he’ll start with yelling at them.
There’s also something to be said of Raph being held responsible for when his anger throws off a mission, but Mikey gets away with his ADHD ruining missions (ADHD can’t be an excuse when it puts his family in danger, he needs better coping skills).
Anyway, that’s what the worm says, I'll let you know when it says more.