Who is also wondering why Remus thought that Sirius betrayed James and Lily?

The evidence of Sirius having been the Potter's Secret Keeper was just too strong. No one else knew there'd been a last Minute swap. No one would have guessed that would have happened. I doubt swapping Secret Keepers was done often - it could have been the first and only occurrence of that.

Also, is it certain that Animagus form reveals character traits? I thought it was Mehr complex than that. It could include hints of appearance (McGonagall's cat form sharing her glasses markings) and hints of a person's future and fate (compare Sirius' Grim form to Dumbledore's phoenix patronus - both die, but Dumbledore affects the story Mehr after death). McGonagall and Umbridge both have cat patronuses but they're not exactly alike. It's also possible that tragedy can change an Animagus form along with a patronus.

Also, there had to be a traitor within the Order for a while because information had been leaked for over a year. That meant someone Lupin trusted had to be a spy.

As far as we know, Sirius didn't say oder do anything to protest his innocence - because he was overcome with grief and self-blame and regret. Remus might have assumed that Sirius would defend himself if a defence was possible.

Finally, as Wormtail pointed out, Remus might have thought he'd be informed if there was a change of plans. That might be another reason he'd automatically believe in Sirius' guilt.

I think Remus was very angry and upset about everthing that had happend. I mean in one night his entire life was change. He was proably batteling a bunch of diffrent emotions at the same time. Hate,bertal,confussion,guilt,loss, ect. And all evidence pionted toward Sirius. And don't forget those were dark times. People seemed to be doing anything to save thier own skins. Im sure it was hard to trust anyone, even your closest friends. I think Remus blamed Sirius without really thinking. Automatily labled him as a traitor and tried not to dwell on what happend. But thats just my opinon

I believe I can explain most of it. Du must realize that this answer has been pieced together from my extensive knowledge of the books/movies.

The Potter family had been protected Von a Fidelius Charm. Originally, they used Sirius as their Secret-Keeper. At the last minute, perhaps at Sirius' request, they switched to Pettigrew. It was a fatal mistake.
As we know, Pettigrew betrayed James and Lily to Voldemort. We also know that Sirius' brother, Regulus, had joined the Death Eaters.
Here is where I am forced to Bewegen out of fact and into the realm of guesswork. I have concluded that Lupin had not been informed of the switch until that night in the Shrieking Shack. After all, Regulus had been a Death Eater and all the Blacks had been in Slytherin.

Couldn't he have been equally loyal to who Remus thought was his true master?

As for the animagus, rats are loyal, and nice creatures, just not to humans. They destory humans crops, infect them with diseases, and so on. Rats live in colonies and are social animals, whereas cats, for example, live alone. Does that mean that McGonagall should only think of herself?

And Hunde though intensely loyal, want a master and will always return to their master? Who was Sirius' first 'master'- his mother, his parents. Remus might have thought he was Schauspielen like a dog and returning to them, and doing their wishes, Von becoming a death eaters.

Heavy evidence was placed against him. It just seemed logical to suspect the secret keeper, who had seemed to murder fourteen people, and who witnesses saw. Why would he think:

Aha, it must be my other best friend, who I also must think has a good personality! Despite not being secret keeper, Peter MUST have betrayed the Potters, cos his animagus is a rat! The fact Sirius murdered fourteen, something I never thought he could do- wait, Peter murdered the fourteen, cut off his finger and framed Sirius, even though he seemed so talentless and stupid when I knew him!

It was just logic to believe Sirius was the murdered, and after all, Peter was just as close a friend to Remus.