That's the difference between criminal and civil liability.
For criminal liability, it is exactly how you describe. Each actor is individually responsible for their actions, the actions of others don't reduce this.
However, for civil liability, there's an apportionment of damages based on a share of responsibility. Say the random YC user sued you, the other poster, and your friend. First, the damages against him would be quantified. This is based on the impact to him, not any moral judgment on your actions. Then, a jury would decide how much of the blame each of you was responsible for, and your liability would be that percentage of the overall damages.
Civil liability works that way because there is fixed pool of "damages" to apportion, so they're working out out how to split the payment. "Responsibility" in a moral sense is like criminal liability, there is no fixed pool to apportion.
For criminal liability, it is exactly how you describe. Each actor is individually responsible for their actions, the actions of others don't reduce this.
However, for civil liability, there's an apportionment of damages based on a share of responsibility. Say the random YC user sued you, the other poster, and your friend. First, the damages against him would be quantified. This is based on the impact to him, not any moral judgment on your actions. Then, a jury would decide how much of the blame each of you was responsible for, and your liability would be that percentage of the overall damages.