It's probable that Rip's trip back the Egypt was soon after his wife and son was killed and wasn't thinking too clearly (and might have had a moment of clarity when he held the knife at Savage's throat and hesitated thinking what would happen if he erased Savage from history). The freezing is a good idea but Snart had already left and Rip might have been worried that more of Savage's cult will arrive soon and their back-up had left with Carter's body.