A seal is a mammal. There is also still debate as to whether fish feel pain, where mammals most certainly do (and killing seals is completely unneccesary).
Imagine if it was a dog rather than a seal, would you still be so nonchalant?
Oh right, so it only matters if it's a mammal?
If like a seal, a dog was relied upon as a staple food and resource for clothing, shelter and fuel, yes I would be as nonchalant. Killing seals is very necessary. You cannot eat them or use their skin when they are alive.
When used properly, the hakapik has been proven to kill a seal instantly and has been deemed "humane". It's no different to shoving a bolt through a cow's head, shooting a rabbit with an air rifle, killing a fish with a priest or breaking a chicken's neck.