Considering he mentioned wanting to do exactly what he did (slit a woman's throat and kill her on his sub) before he ever met wall. I would go with it was premeditated. "They heard how he had joked about “a murder plan” involving tying up a woman on his submarine and slitting their throat with a knife in a set of messages sent to a friend, which he had subsequently tried to delete." Note, he tried to delete these.

He also changed his story multiple times. Beginning with dropping her off on shore alive, to dropping a hatch on her head and finally carbon-monoxide poisoning.