For good people to do bad things...

[Ryft]

For me, and indeed most people, the definition of Christian is a person who accepts Christ as their Saviour.

Are you recognized as an authoritative source on what constitutes a Christian? Nope. Q.E.D.

All Christians universally recognize one source as authoritative on what constitutes a Christian: the canonical Scriptures. There are differences over the qualified authority to teach religious doctrine, but the source of that doctrine is universally recognized. And it's not you.

Now your one sect of Christianity might have one view on the details of Christianity that another Christian sect does not share, this does not make the other sect not Christians.

According to...?

If they then commit a terrible act that accords with that sects belief because of that belief, then the act was the fault of religion.

As I already said (and please don't ignore it this time): "To conclude that the religion is the cause, it must be shown from the religion itself. A conclusion about the religion which is based on the character or circumstance of its adherent is fallacious."