Could Obito have stoped the uchiha massacre???

[Demonic.]

Madara never told Obito to attack Konoha though. And Obito never thought of Madara as a comrade. He never followed Madara's plan, even if Madara gave him the idea.

Get slapped back, Father.

Just because it was never stated doesnt mean he didn't. Madara told Obito to "live on as him".

-Madara tries to attack Konoha with Kurama
-Obito tries to attack Konoha with Kurama

After that event some people like Jiraiya, Minato, and Itachi suspected Madara had attacked because they assumed he was the only one that could control Kurama. This fit perfectly with the plan - Obito pretends to be Madara to scare the 5 nations into submission.

Bold is irrelevant. He still followed Madara's orders.