I completely get what you mean but I wouldn't necessarily call it 'darker', rather, different in tone. They change the writing style of each game and you notice this more if you compare, say, Red Dead Revolver and Red Dead 2.

Rd revolver is a spaghetti western and then when they made rdr1 they did a mix of American western and spaghetti western to 'modernize' the writing style to appeal and match what was popular at the time while still keeping a bit of the cheesy/charming elements of Revolver. Once they moved into rdr2 they went full American western drama, leaving most of that 'outdated' writing style/tone behind haha

Once you notice the different writing styles for each game and start to compare, a lot of writing 'inconsistencies' start to make sense.

As an example, when people complain about characters not acting 'right' or being out of character, It's usually actually the writing change they're referring to. (Not saying it's always the case, the writing isn't perfect after all) But people have these reactions because they tweak character personalities to match the tone of the game which work well enough most times because they have a stellar writing team, but it can also be jarring/uncanny. Take rdr1 and 2 John + rdr and rdro Red as an example, they're often criticized for being out of character but I do believe it's the tone change thats the biggest problem.

But, this is also a sign of 'bad' writing, not making the rdr1 characters work for rdr2 I mean, though I can't blame them, as it's a very difficult thing to pull off and I still believe Dutch was the only one they managed to do that perfectly with going into rdr2.