This doujin is really...uh hard to find the words to express. Interesting. I'm surprised the artist doesn't make this and sell this professionally. The art and creativity is there.
At least two reasons come from official sources (interviews and such, seek in the puella magi wiki): 1- the author wanted to show that QB and humans are completely different and unable to understand each other showing this alien reaction to the death of a comrade. 2- for unknown reasons QB does not want to show its immortality power and so destroys the evidence eating the previous body.
Of course the author is also famous for being a liar... so...