To bring balance to the force he destroyed both the Jedi and the Sith. The Jedis thought bringing balance would mean destroying the Sith but it was pretty clear that the one who would bring balance to the force was both a jedi and a sith. Aside from stupid plot things like Anakin was Space Jesus and medicloreans there's a story there. The reasons for him joining the Dark Side is a bit cliched and almost like he was forced into it because he stopped Windu from killing Palpatine because he thought he would teach him how to stop death, which Palpatine did not know and Anakin only discovers how when he returns to the light side as in the end of VI ( this is hinted somewhat at the end of episode III I think when Obiwan goes to Tattooine because Yoda tells him something like his old master is waiting there for him, could be wrong though). I think he stays because of the power and I guess believes he could kill Palpatine but then goes batshit crazy and ends up causing Padme to die and has a longass fight with Obiwan which ends with him nearly dead puts a halt on his plans and why he is looking for his son to take his place and kill the emperor.
I could be wrong about this though I haven't watched episode 3 as much as I have with the old movies, it feels fun to talk Star Wars though reminds me of fun nerdy times. Excuse if this is difficult to read , if it is I will try to edit it. Yeesh I wrote a lot, f--- it.