Rachel saw Allison chopped up into tiny bits and immediately knew that this wasn't an accident otherwise he wouldn't be so swift to take this action.
It was premeditated and Jason had killed her in cold blood, there was no doubt about that.
She no longer knew the man she was in love with and she couldn't help but throw up all over the floor.
She had to leave the house before Jason returned but he would know someone was there because the vomit got all over the bag.
Rachel didn't know what he was capable of anymore and despite her mind logically trying to find an excuse for why he did what he did, she couldn't.
Jason was the one person who inspired change but they didn't take into account that he too was subject to change.
Allison was dead and there would no doubt be questions surrounding her disappearance.
This would bring unnecessary attention towards them and she couldn't risk it.