Bokusatsu Tenshi Dokuro-Chan

The story is about Kusakabe Sakura, a second-year junior high student who, in the future, discovers the power of immortality. An angel named Dokuro is sent to kill him before he does so. However, Dokuro thinks Sakura can be spared from this sad fate, thus, she keeps him. Another angel named Sabato is later sent to finish Dokuro’s dirty job. Pipiru piru piru pipiru pi!
From Baka-Updates:
From AnimeWaves:
Pi piru piru piru pi piru pi~ Along with this mysterious melody, an “angel” comes to Kusakabe Sakura-kun’s house and beats him to death. Her name is… Bokusatsu Tenshi Dokuro-chan!? From now on, the everyday fantasies of Sakura-kun’s life will be replaced by events more strange and interesting than he could ever imagine…
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In the future, a man named Sakura is destined to create the technology for eternal life, which (due to Sakura’s lolicon tendencies) freezes all women’s aging once they turn twelve. God refuses to allow this, so he sends his angels to the past to kill Sakura before he can do this. But one angel, Dokuro-chan, decides to try to change the future without killing him. Problem is, Dokuro is very temperamental, and is constantly killing Sakura (and lots of other people too) in incredibly bloody ways with her giant spiked club. Good thing she can bring people back to life – if only to grusomely kill them again the next time she loses her temper.