So here we have the climactic moment that will send Ichabod hurtling down a spiral of despair and is very important for his character arc.
Donatello was starting to feel like himself again, he had gotten out of those embaressing clothes and had a purple mask back on, now he was desperatly serching for a substitute for his Bo staff, a thin piece of metal pipe would do, but as he was searching these catacombs connected to the sewers all the pipes he could find were much too large to fill the place of his staff, Don sighed remembering that sometimes they had to work with different weapons as Master Splinter had taught them to do and he was about ready to say 'Shell with it' and use a piece of a giant pipe as a weapon but then the sudden ringing out of a familiar voice made him nearly jump out of his shell.
"You Trecherous Feline!"
That was Ichabod's voice all right, but who was this 'Trecherous Feline' he was talking to? Surely not Marzipan! Using his Ninja Stelth, Don crept gingerly towards the sound of the voice, and peeping around the corner he saw Ichabod from the back, he was talking to a Black Cat-Boy. Now Don could have revealed himself, jumped into the fray, but out of respect for the human boy growing into a man, he wanted to let Ichabod prove he was able to fight his own battles Don felt he should only intervene if he absolutely had to, but this Cat-Boy seemed to be Ichabod's own age and seemed to be not much larger in terms of meat on his bones, surely Ichabod could handle such an evenly matched opponant!
"You deliberatly sent me into an ambush!" Ichabod fumed at the smirking Cat "I could have been killed!"
Kevin didn't say anything but the tip of his tail started to twitch.
"Why?!" Ichabod fumed "Are you really so in love with Marzipan, a Girl you've never even met?!"
"Because she's a Cat and you're a human!" Kevin finally snapped "After all I've been through, after all I've seen, humans and Humanimals in love disgusts me! So I've decided this line shall not be crossed! And I'm telling you Human...I will take Marzipan away from you and prove to her she's better off with her own kind! You can't stop me, I have my will, Marzipan will be mine!"
Don did feel a shiver go down his shell with the amount of conviction in the Young Cat-Boy's voice the way he proclaimed he will do these things and that nothing could stop him, either the Cat-Boy was delustional or Don feared that he might also have some special power...
However he couldn't see Ichabod's face twisting into such terrifying expressions they would have haunted Don for the rest of his life if he could see them, Ichabod really did seem like a ghoul at that moment!
And with the rage inside him pent up like boiling lava...Ichabod jumped up and screamed out his fury! And more than that like a cartoon character he was actually on fire! Don would later learn this is an actual thing that can happen on First Earth, 'An Emotional Combustion' and while it often results in first degree burns the People of First Earth view it as much safer than an aneursym.
But because that fire and smoke was real...As Ichabod collaped all singed...His smoke set off the fire alarm...
Yeah, this was inspired by the scene when Twilight in MLP 'Evolves into Rapidash' and just taking the classic cartoon gag both literally and seriously if such extreme moments of ager could cause literal fire and what such consquenses of that might lead to.