Intimidation
With all that had happened, Seth figured leaving without telling Irene was the best solution.
With how chaotic their relationship had been the past days, acting like friends or lovers one second and staring at each other like total strangers the next, Seth didn't have faith in Irene maintaining her calm were she to know he planned to depart for whatever reason.
As for the matter of them taking the soul fruits and running off, Seth had ensured that didn't happen by including the clause in his contract with Aimy.
The signing of the death contract had been a new and weird feeling for Seth.
It felt like he had opened up a portion of himself and then let it be infested by some foreign power, and until the contents of the contract were fulfilled, he would live with this infestation for eternity.