Donatello had gone insane, he brutally killed his brothers by putting them through insane experiments. He was currently putting Master Splinter through a terrible inflation test, Donatello hadn't caught you yet but he knew he would. All the exits and entrances to the lair were blocked off using his amazing tech, that's how he did this all in the beginning

After the test killed Master Splinter, Donnie began looking for you smugly. His hands and face covered in blood splotches, he slowly entered the room you were in. He didn't know exactly where you were in the room but he knew you were in there

"Come out, where's your pretty face hiding? Come out, I don't see any point in lying" Donnie sang quietly as he slowly walked through the room, looking under the bed and lifting the covers of the bed. He was singing because he was dramatic and liked the chase

"You have questions I can answer, but first come out with your hands up" Donnie sang again quietly, walking to the closet and opening it to no avail

Come out, where's your pretty face hiding? Come out, I don't see any point in hiding" Donnie sang again, a little bit louder as he looked under the desk to no avail

"You have questions I can answer, but first come out with your hands up!" He sang again, walking to the middle of the room and standing there. He was waiting for you to come out and give up