This is the fastest ending and it doesn't need any special conditions to unlock. When chatting with Johnny on the roof to decide what to do, start by selecting one of the options available to you. When Johnny asks if you are sure, another option appears, that of proposing to end it immediately. Then validate the option. It's not very cheerful, but at least you unlock a reward without a lot of effort and wasting time.
The other alternatives involve one last big mission, each version is different even if there is always the same final boss.