"You-- You MONSTER!"
This moment is super important for two reasons.
Firstly, it emphasizes the whole parallel between the Shredder and Splinter and Raph and Leo. Raph’s looking at this horrible man who likely just killed his father, and he thinks monster, because it takes a special kind of terrible evil to do something like that, right? But, given the right circumstances, and if Raph relished and embraced his rage instead of trying to control it, could he turn into someone like the Shredder? If he pushed his brothers away for good, if he didn’t have his family the way he does, could he be like that, so vengeful, awful, obsessive over strength?
The answer is yes. Yes, he could.
(Shredder even talks about how “his rage makes him strong”, and all that. WHERE HAVE WE HEARD THAT BEFORE, FOLKS.)
Also, Raph may be the physical strength of the team, but Mikey? Mikey is the emotional strength. He’s their rock, so to speak. Besides him being their brother, it’s why the others need him— and it’s also why he and Raph are so close. Mikey is empathetic and sensitive, while Raph is not; Mikey can easily deal with, handle, and express emotions, while Raph has trouble. So it’s kind of like Mikey is there to do it for him, and he knows just what Raph needs at a time of such turmoil.
Arguably in this episode more than any other, Mikey and Raph’s relationship is shown in-depth. It’s really reminiscent of the comics, where Raph was closest to Mikey out of all his brothers, and I think it’s definitely true in this version, as well.