Lots of fighters, even close friends or brothers as you pointed out, are willing to fight each other. If you think that because you are close to some training partners that somehow makes it ok you're wrong and demonstrably so since lots (most) other fighters are willing to do their job.
If you can't understand something as simple as separation of job and friendship then don't go into MMA, period. There are lots of jobs where the issue won't come up, do one of those.
You like to describe it in such basic terms, like MMA is the same as any other job. You're wrong, it is not. MMA athletes seek to inflict damage to their opponent through combat, they are not working in a local shop.
It doesn't matter if some fighters are willing to do it, that means they probably are less bothered about the damage that can be caused in a fight. The dangers of MMA are very real, and when you've got hands like Anderson Silva or Paul Daley you know you could leave a best mate unconcious on the mat.
It doesn't matter how much you simplify it, how much you compare it to regular jobs or other sports, MMA is an entirely different beast with a high KO ratio. No amount of training or money changes the fact that brain injuries in particular are serious business.
Don't bother saying they shouldn't be fighters if they can't accept the dangers, because again that's just a pointless oversimplification. Dangers involved when you don't know the other guy and he wants to smash you are different to dangers when you are close friends with the other guy and neither of you want to do it. It becomes a very uncomfortable situation.
The concerns of damaging someone permanently, on purpose, cannot just be ignored by some people. You really should understand this.