Idealism
Finn's death didn't strike him hard. He kind of saw it coming.
The real blow was Niklaus. He just couldn't grasp that Niklaus was somehow deficit, that he had a flaw, that for once he didn't have a plan B, that Niklaus was still mortal.
In the very depth of his mind, he knows that Klaus may have found another loop hole , that he managed to cheat death, after all the whole blood line is still intact, save Lockwood.
Right now, he can't process well; he needs time to compose himself so he could delve into it properly.
He knows he is the reason behind this mess, had he stayed, he could have sorted the whole ordeal out.
But he was so moral, so reluctant to stay and cause trouble to the poor girl. He left out his family in the time every single one of them including Finn and Niklaus needed him.
He resolved "I am a coward and a traitor"
Now that he is witnessing the consequences, he knows that what he did to Ele- no the dopplegagner was justified, after all she sold him out first.
Nothing new, Replica of Tatia and Katerina.
Had he stayed, he would have eased some of Rebekah's insecurity and uncertainty; he would have dragged Kol Back.
Had he stayed, he would have protected Finn.
Had he stayed, he would have helped Niklaus, numbed some of his uncertainties, some of his abandonment issues and his fear of loneliness.
Instead, He walked away from them.
He was just good at criticising Niklaus' unorthodox methods of handling obstacles and troubles.
Never had he the courage to try to solve them first.
He would only intervene to clean up the mess, or sort it out after his brother complicates it more.
His conscience used his brother as a refuge from guilt and immorality. It was always Niklaus' inhumanity, Niklaus' lack of morals, Niklaus' lack of conscience or feelings.
It was always Niklaus rolling in blood.
He had phoneyly chastised himself using Niklaus.
This time he couldn't.
Niklaus is gone to god knows where because of him.
The one time he had taken matters first hand, his faked, shattered morality showed up and he couldn't face the aftermath, and he couldn't bear it.
Ironically, the unholy Niklaus was the only one who never left.
The twisted Niklaus kept them together, as possible as he could.
The implacable Niklaus was the one who let go.
Niklaus let go of Tatia and didn't succumb to the dopplegagner allure. He didn't.
He let go of his mother's rejection and was willing to give her a chance.
He would have let go of Mikael, except he was always hot on his heels.
He couldn't let go despite knowing Klaus was blabbering threats in the Katerina situation. He chose to condemn Niklaus and Leave without even trying.
Even with all the guilt and the pools of blood he had shed, Niklaus was the ony one chasist.
Elijah wasn't.
Elijah, the single dimensional, with the strong sense of responsibility and humanity wasn't.