There are a number of good thoughts as to why MIB killed Seth Norris, and although we cannot actually prove any of them wrong, I think its safe to say that several of them together could very well be right.
The Man In Black, or the Smoke monster has claimed many victims for many reasons. The ultimate goal and reasoning for his violence was to leave the island, something he was willing to achieve at any cost.
He was a man first, and the first victim we know of was the one who raised him, and it was out of revenge and anger (Flashback to a knife in Mothers back).
He killed to help himself in the long run, sometimes while others thought he was helping them (Flashback to Ben summoning “Smokey” to attack the freighter crew).
I believe the main reason that MIB killed Seth Norris was because of someone who was not even at the scene of the murder…John Locke.
The first thing that had to be done was eliminate the only man who has any clue as to the whereabouts of their current destination. Location equals a way to chose a direction off the island before he is able to “embody” the correct…look for the occasion. So very many things need to happen before there can even be a thought of John Locke leaving that island.
This plot…this search for the loophole has been in the works for a very long time, and John more than likely has been involved since at least the 1950’s I would imagine. Around the time when John Locke was first on the island from a chronological perspective by dates that we have learned.
Seth Norris, like Jonas Whitfield, commanded a vessel that “accidentally” crashed onto our island…and like Jonas Whitfield, met a similar fate (Flashback Richard Alpert witnessing Jonas Whitfield being ripped through the ceiling of the Black Rock’s prison cell).
“Six hours in, our radio went out; no one could see us. We turned back to land in Fiji. By the time we hit turbulence, we were a thousand miles off course. They’re looking for us in the wrong place.”
Without the pilot to assist their location, they all became truly LOST for the first time…non-metaphorically of course.
It’s quite obvious The Monster could kill at will, with no mercy (Flashback to Jacobs protectors in the statue), and sometimes no real reasoning (Flashback to Danielle Rousseaus team). It easily could have killed anyone including Jack and Locke…but instead actually showed them both something.
MIB admittedly posed as Christian Shepard and led Jack to water…not to mention the empty casket his dead father was supposed to be in. MIB is built on deception and mind games.
He also showed John Locke something.
“”I looked into the eye of this Island, and what I saw… was beautiful.”
John told Eko it was a ” very bright light” that he saw, to which Eko disagreed with seeing.
Eko being another example of the monsters power and perspective. He could read Eko (Flashback to the first time we truly saw the monster flash and scan) and know his deepest sorrow, his biggest weakness. MIB used Yemi against him just as he used Alex against Ben. He realized Eko wasnt going to play his game, and just as he did Zoe (Flashback to the throat slit scene…maybe watch it twice), eliminated a game piece because it was “pointless” to him.
He even took the Losties to the plane after freeing them from Widmore’s cage so close to the end (Flashback to John Locke shooting and neck breaking his way to the cages and into the Ajira plane to take the explosives that Widmore wired)
Some believe that Seth was killed out of his uselessness…which is possible…
But Seth Norris was not always pointless…not by a long shot. He is the one who brought them to the island, and he is the one who was the first example of his ability…a flex of his muscles if you will. But it was Seth’s possible idea of the islands location that was his biggest fault most likely.
Like his brother, MIB knows the people that are important. He knows that certain individuals traveled through time. He showed us this by his knowledge of the compass and its importance of helping John Locke stay on the right path of following his destiny. He knows what John knows, and I would imagine what Christian Shepard and MANY others alike knew.
The Man In Blacks attack against a “pointless” character such as Seth Norris was a way to show his strength, and rile the cages of a few of the important players in Jacobs game…after all, we know he cannot directly kill any of Jacobs candidates, or at least he follows that rule from a technical standpoint (Flashback to Sawyer pulling the bomb wires on the sub).
The Man In Black’s attack against the Pilot, Seth Norris was to take away the opportunity for John, Jack, and the rest of the candidates to leave before he was ready…he may have still been contemplating “what to wear”. He may have been waiting for his own candidate to ripen.
The Man In Black may have killed the pilot for many reasons…but the principal and foremost reason was for us, the viewers.
Not to jump when “a monster” rips a man from the cockpit of a plane right in front of our hero’s eyes.
Not to fear for our Losties as they run through the jungle from the beast as it seems to engulf them as it makes its approach.
Not even to make us wonder 6 years later why he just didn’t allow them ALL to meet the same fate as the Seth and the “Temple Others” (Flashback to The Smoke Monster in full wrath mode killing everyone who didn’t take his offer to “leave the island with him”).
In the pilot episode, the end of the very first one…he didnt kill the pilot more for any reason than to make us ask the very same thing that Charlie asks as he, Jack, and Kate look up and stare at what reflects in the puddle below, “How does something like that happen?”
(Flashback to the white shirted, wingless, and bloodied pilot hanging in the tree)