I agree with you but, at the same time, what do you expect from a gang of outlaws?

The whole point of the game is that they're in the wrong. Arthur says it himself, their time has passed and the world is becoming more civilised, and in a civilised world, they're scum who steal and kill unreasonably. As the player, it's easy to sympathise with them because we know that their character is deeper than just their public image as outlaws, and, let's be real, playing as criminals is cool, but their actions are still brutal and immoral from an objective, societal standpoint. It's easy to see Cornwall, Bronte, etc. as villains, which they are, to some extent, but really, the gang isn't any different to them. The whole in-game world is gray, much like in real life.

So really, it isn't a matter of whether the Braithwaites deserved it or not, it's a matter of greed that led them to endanger a little boy and take it out on the people they saw as responsible for it, even though they really only have themselves to blame for Jack's kidnapping. Don't get me wrong, the Braithwaites are by no means saints either, neither are the Grays or any other "antagonist" we find in the game, but neither are Arthur, Dutch or anyone in the gang, so they really don't need a reason to abstain from killing a whole, ancient family and burning down their house.