why were they friends with Francis

Sometimes it just happens that way.

Plus I think it was out of fear more than friendship

I don't know how they know him in the movie, given that he's obviously older than them, but in the books, they went to school together. The gang liked having their own pet nutter, until it got to the point where they were scared of him.

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It's shown in T2 that Renton went to school with Begbie, and Begbie was older but held back for being stupid.

There's a direct reply to this question in the movie when Mark says "He's a psycho but still a mate" or something in that vein (pun intended).

Do you have any tobacco?

They've all known each other since they were little boys at primary school. In the novel it's clear Begbie was a pretty normal kid until he started to believe his own publicity when they were all about 14 or 15. That's when he starts to become violent.

"There is no more human race. There is only.. the Master race!"

They were all cowards. If they ever got ripped off in drug deal they would turn to Begbie to help get the money back. All junkies have that one tough pyscho friend they can use as heavy when needed.

If you ever meet any junkies you'll see what I mean.