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[Heroes] How does Sylar steal powers?

So, the character Sylar cuts open peoples heads and take their powers. When we see him do i on Claire, he scrathes a bit of the brain, but it's not really explained. Also, why does he steal the brains after he's done?
gpt
Sylar possesses a power The Company has come to call "Intuitive Aptitude": the ability to understand something perfectly just from studying it closely enough, to the point of being able to copy it fully.  He can learn anything by studying it closely, including how to use a biological mutation.

Intuitive Aptitude is actually a more evolved form of Empathic Mimicry, an abilty once posessed by Peter Petrelli.  Sylar does not realize this initially however; the ability to understand everything creates a deep desire to know everything, which results in him taking the most direct route: killing people so he can examine their brains directly.  Once he examines exactly how their brain is altered (to allow their power to work), he can use it to its fullest extent.

Since his ability is a more evolved form of Empathic Mimicry, he can still use an empathic connection to access someone's power, but he won't be able to use it on a level of understanding as if he had studied their brain. 

However, empathic connections are much stronger than simple learned knowledge:  this is why, even after having his powers "muted" by The Company's efforts in the past, when he regains his base ability, he still has access to Telekinesis.  Although he did study Brian Davis' brain to originally learn the power, it was his first kill.  The raw amount of guilt he felt over killing Brian after just learning his name and ability actually formed an empathic bond to the power, not a simple knowledge bond.

He most likely steals the brain just in case there's someone else like him (in his first encounter with Peter, he recognizes that their abilities work off of the same base).  Beyond his power, Sylar has a drive to be unique - "Special" - and that means he doesn't want others having the same abilities he does.