Jason's mind was in turmoil as he went back to the beach, but it wasn't because of his earlier actions – he had already come in terms with killing since – he was trying to create a lie so good that they would believe him.
By they, he meant the other awakeneds that were in this dungeon.
Even though practically everyone ignored the words of the elder when he listed the rule that said 'no killing', if they found out that he killed not one, but six people, the awakeneds would obviously report to the elder.
Jason was the only person that had gathered over fifty hearts and it was obvious that there would be a lot of people that wouldn't be happy about it.
The simplest approach he could take was killing all of them.
But it would be surprisingly if only he made it out of the dungeon, when 30 other people also entered with him.
Trying to give the only other character that I've introduced that is friends to Jason character development