Bludgeoning Angel Dokuro-chan/WMG

Dokuro-chan and the Angels are all Counter Guardians.
In the future, Sakura's invention will somehow threaten human existence, so Dokuro-chan was sent by God/Alaya to assassinate him. However, she proved to be too nice and instead just tried to change the future by preventing him from inventing immortality. The reason Zakuro looks older than her but claims to be only 9 could be explained by the "ages" referring to their tenures as counter-guardians, with Dokuro having worked for 14 years while Zakuro worked for only 9 and made her deal with Alaya later than her big sister had. Either that or Zakuro was a homunculus in life.

Dokuro-chan takes place in the same world as Panty and Stocking With Garterbelt
Like Panty and Stocking, Dokuro is a violent angel who uses unconventional weapons and has no regard for human life.
 * While entertaining, it is impossible. Angel weapons don't work on humans in Panty and Stocking. Exalibolg.... does.
 * Considering the fact Dokuro was sent specifically to kill a human and Panty and Stocking were sent to kill demon ghosts, the two could easily have been given drastically different kinds of weapons.

God knew what He was doing when he sent Dokuro.
God, being the omnipotent that He is, didn't just want to prevent Sakura from inventing immortality; He also wanted to make Sakura suffer as much as humanly possible. Dokuro is not just a ditz with an oblivious mean streak; she is one of the cruelest people in the multiverse. There's just no way that she couldn't know what she's doing as she meticulously destroys every little bit of Sakura's life. It almost makes you wonder what sort of standards God has for His angels.

Sabato is the renegade angel: she genuinely feels sorry for Sakura and wants to permanently end his suffering.


 * Alternatively, God knew that Dokuro simply would not understand what she was doing to Sakura. After all, he made her that way, just for this reason.


 * Alternatively, Dokuro is an Ascended Demon combined with Boxed Crook. Yes, Dokuro is probably a Complete Monster. God commanded her to punish Sakura because she's like that.

Sakura is dead and currently in purgatory.
Dokuro did succeed in killing him the first time they met. The first time she "resurrected" him, it was to transport his soul to a custom-made purgatory where everybody hates him and his "best friend" is a sociopathic maniac.

Sakura ends up inventing immortality because God sent Dokuro.
What better way to spite God for sending a cute girl to destroy his life than to show him that even he cannot change the future?
 * This was God's plan all along. It turns out that the entirety of human existence is a set up so he can get sick sexual kicks out of watching the psychological effects of a culture having to shift over from seeing sex with twelve-year-olds as horribly immoral to sex with twelve-year-olds being the only thing on the table.
 * Just as planned? So, is God Kira?
 * No, he's a shinigami desperate to avoid losing his job after immortality is invented.
 * And a jerk, if Shin Megami Tensei is to be beleived.

Dokuro and Sabato's presence gave Sakura a terminal fear of anything with boobs
He added the detail of females staying extra-young to his immortality whatever on purpose, just as Dokuro said - not to cater to sexual tastes, but to avoid the memory of the horrors he was put through by divinely-armed pubescent girls.

Sakura didn't ensure that all girls stopped aging at twelve for the loli appeal; he did it because he realised the effect a fertile population of immortals would have.
Think about it. If you never grow old, then chances are you'll want kids eventually. But then you're still there when those kids are having their kids, and so on for as many generations as the planet can sustain. After that comes a mass die-off, or else hell on earth for zillions of immortals. Sakura simply allowed a choice: you can live forever or have children. Pick one.
 * Does everyone have to make that choice, or only women?

Some angels age quicker than humans.
Well, it would certainly explain Zakuro.

Sakura isn't the one who invented immortality, and the entire show is a horrible case of mistaken identity.
Dokuro and the others were products of Sakura's research into immortality. Therefore, if Dokuro succeeds in preventing Sakura from inventing immortality, then she should stop existing because she's the product of a future that no longer exists. Therefore, since this doesn't happen, Dokuro is tormenting the wrong person. There's another 12-year-old Japanese boy named Sakura who's going to invent immortality.
 * Unless this is one of those causality things.

God is entirely aware that Sakura has nothing to do with the discovery of immortality.
God's just sort of a jerk.

The entire plot is the result of some Jerkass demon hacking into Heaven's computers For the Evulz.
Why would God send an assassin? I think that a demon managed to spoof messages from God just so Dokuro would be sent to torment Sakura. As for why this demon would do this? He's a jackass, he does it For the Evulz. If this guessing is right, then when Dokuro finds out, she'll be pissed .

Sakura is the Reincarnation of Kenny or visa versa.
Come on! It explains everything! The dying, apparently being perverted, the buttmonkey status... everything!

Sakura-kun is a huge masochist
He just doesn't realize it. This explains why he misses Dokuro-chan so much at the end. Plus why his most serious reaction to being turned into gibs is "That was mean!"

Sakura descends from Keiichi, Geek-Boy, Kenny, Meg...
and every Butt Monkey you can think about. but the four above are the mainly blood-related to him.

Sakura goes to help Sabato not because he's an idiot; he goes to help her because he's become suicidal.
Honestly, it makes sense - after all that Dokuro has put him through, Sakura just wants to end it all. He just is unfortunate enough to try Suicide by Cop on someone who's completely incompetent.

Sakura steals the youth from Heaven in the future.
It struck me as odd that all angels are all young girls, just a bit older than the little girls in Sakura's future. But then Zakuro appeared, younger than Dokuro and yet much older-looking. So what if Sakura creates a device that takes the youth from angels and gives it to humanity? Since Zakuro is from this future where angel youth is being harvested, she ages much faster than the others. God sent hit-angels to prevent this because he no longer has his harem of unaging lolis, which fits the tone of the series.

Sakura's classmates aren't subject to the Weirdness Censor; they ignore it actively because they fear for their lives.
What happens in the first episode, when it looks like Dokuro isn't going to get her way? She transforms the class rep into a monkey. She also very quickly proves to be extremely deadly but only willing to revive Sakura. They know just how weird everything is. They just think that acting like it's no big deal is the best chance they have of survival.