SBM.
I get that his actions after the "event" are unexplainable. But if it was all CYA, then he did a terrible job of it--not taking her phone or purse, not cleaning up the house, leaving sheets in the trash can, not having a plausible explanation for her disappearance.
My only explanation is that he disassociated himself with the act, went into shock, and everything that occurred after the violent event was zombie-like, almost in a trance, autopilot.
I suspect that he may have intended to commit suicide (like 81% of other family annihilators), but he didn't get a chance to finish what he started. He was caught off guard by NU and never got a chance to go back to the tank site to take his own life.