Why did Su Ming say that? Because the two of them approached things differently.
K investigated according to the patterns of past homicide cases, hoping to use clues to find a serial killer.
Taking organs or clothing from victims and believing oneself to be connected to the police were all typical signs of psychopathy.
So K believed the killer was a "stalker" type. He must have followed the victims and been drawn to certain traits they possessed, which led him to commit the crimes.
Deathstroke saw it differently. He believed this was a way for a High-Dimensional Being to greet him. The victims had only one thing in common: they were all Aliens. Other than that, there was nothing at all, making it more like random killings.
This High-Dimensional Being might be quite young, because it did not even have its own method for killing Aliens. The two crime scenes had respectively imitated the Green River Killer and Hannibal. One was a real case from another Earth, while the other was a fictional character. This was a typical trait of a copycat serial killer.
Based on his earlier conversation with J, it could be confirmed that this Earth had no stories about Hannibal. He was not even a character anyone had heard of in Superhero comics.
K had seen both crime scenes, yet he made no connection between them. With his capabilities and intelligence, if he knew of the Green River Killer and Hannibal, he would certainly have noticed the imitative traits.
But he had not, which meant that stories of those murderers did not exist in this Men in Black universe.
It made sense. The first Deep Space Gate had appeared on this Earth in 1889, in Paris, France. At the time, the renowned Architect Eiffel had formed the