It's all of that.

First of all, Sufficient Assumption is a real front-loaded question. You SHOULD spend way more than average time on the argument and way LESS time on the answer choices.
By the time we reach the answer choices, we already know what we're looking for, so we can just scan for what we're missing ... we don't need to read and consider every answer choice.
In some cases, diagramming is really critical because otherwise it's hard to see the conditional chain created.
In this example, it's more debatable. Some people will definitely put that last sentence together as they read.
If you don't believe you're responsible, you stop feeling ashamed, and so you get a general moral decline.
Since the conclusion said "XYZ would get you a general moral decline", we could anticipate that we need to connect "XYZ" to "don't believe you're responsible".
In this case, XYZ is "everyone accepting that we're just natural objects subject to natural forces".
A couple things you could check yourself on for this example:
1. When you hit the "after all", did you realize 'Well, I guess the first sentence was the conclusion. Maybe I should re-read it, since THAT is the claim I have to prove.'
2. After you've re-read the first sentence and thought about the meaning of what they're talking about (i.e. that we see ourselves like animals / rocks / stars ... not supernatural souls with free will), the 2nd sentence says "After all, if we don't think we're
responsible for our actions" ...
That should feel like a record scratch that brings the conversation to a halt.
Wait, wha?
Who said anything about "not believing we're responsible for our actions"?
The students I've seen who mentally intuit the answer to this question without diagramming are almost always thinking about the actual subject matter of the first sentence and hearing that "disconnect" with where the 2nd sentence starts to take the conversation.
All that being said, although this question can definitely be done without diagramming, many Sufficient Assumption questions really ARE easier with diagramming and you basically need to get a lot of reps when it comes to quickly symbolizing / linking conditional claims.
The good news is that once you get so good at doing so that you can do so relatively quickly, your brain gets so comfortable with conditional logic that it starts to be able to do it more mentally.
So you need to practice writing it out a lot early on in order to earn the second nature awareness you might eventually reach, at which point you DON'T need to write as much stuff out.