Deidara didn't want to fight Sasuke, so to speak, it was just his job to kill Orochimaru for Sasori. Sasuke had killed him so he had to kill Sasuke.
But Deidara wasn't going to win this fight.
His cheek stung where He had just been punched, his mouths ached, he sat up ignoring the pain. His sharingan was deactivated, even then he showed no sign of fear, Deidara hated him. He hated his brother too, the reason he was here was because of Itachi.
He wished he had fought harder that day, and won against Itachi then he would be free, but he hadn't he underestimated his power. Deidara looked down he didn't have much clay left, or chakra there wasn't much he could do without killing himself.
Emotions swirled, thoughts ceased, he realized he didn't have a reason to live anymore. He never had a reason to fight besides for his art, but now he knew a way to make sure everyone would know of his art.
He grabbed and the hem of his shirt and ripped it off, the cool air stung his open wounds, he tugged at the stitches covering his mouth, painfully removing them. He smiled people from miles away would be able to see, his art would fill them with awe and fear, introducing them to new life, showing them what true beauty is.
That was what he wanted now, to teach the ignorant how they have been living there lives wrong, and how they can change it. This blow would make a impression on the earth, one it wont recover from.
He shoved the clay into his chest mouth, he had never been happier, this was truly art, his dyeing would give birth to new artists, to new live, keeping his art from fading into oblivion.