Nobody mentions this, but here is the best way to unlock both endings in one run. So in the final cutscene, where Ethan orders Malcolm Vanhorn to stop the car, you can either choose to kill Leland or let him leave. After you gain control over the character, you have 5 seconds to decide. Pull the trigger to obtain the following achievement:
Now after the achievement pops, immediately press Start and choose the Restore Checkpoint
option. Unfortunately you have to fight against the Hate, but only the third phase. Watch the cutscene over, but this time do not kill the SKX. You'll got the other achievement as well:
This way you don't have to replay the last chapter all over again.