@Strawberry Shortcake said:

Could anyone please explain who was that prisoner, and what does that scene have to do with the movie itself?

Not having seen the movie, I am assuming by context that if there was a 'prisoner' then it was probably Cletus Cassidy - Carnage.

He pretty much said something along the line that when he escapes, it'll be "Carnage"...plus to us comic book nerds, the blood writing on his cell & that he's a ginger (love Harrelson in a Little Orphan Annie wig..lol) makes it quite obvious its Cletus Cassidy.