Anakin fell to the dark side way too quickly
OK, so Anakin's really close to Obi Wan. They're like brothers. He's a really good person, fighting for the Republic and helping people. He all cool.
Then Palpatine begins talking to him about this sith lord that could prevent people from dying. Anakin is clearly interested.
Anakin then learns that Palpatine is really Darth Sidious. He tells Mace Windu who tries to arrest Sidious. They duel, and Mace defeats Sidious. Anakin inadvertently helps Sidious get back and kill Mace.
Anakin regrets what he did, but then just kneels down to Sidious, a SIT H LORD, whom he's not only been taught to hate (for good reason), but has also had bad experiences with them, and BELIEVES Sidious's claim that they can learn to save Padme, and actually BELIEVES that Sidious cares.
So then SUDDENLY, he goes and starts slaughtering younglings and Jedi whom he used to fight alongside. He seems to hate them all the sudden for NO reason.
He also suddenly hates Obi Wan for NO reason, and hates the Jedi for NO reason.