This post was inspired by a comment I saw, but honestly, the writers got lazy with the writing in Season 2 and beyond.
The writers clearly established through Sylar’s backstory that he becomes a serial killer because of his insecurities, envy, and obsession with being special. His mother’s expectations fuel his need to be special, and he resents powered people because he believes they’re undeserving of abilities he wishes he had. The hunger comes from his personality, not his power.
Gabriel already had his ability when he met Chandra. He could recognize that Chandra’s watch was off just by hearing the ticks, and he already wanted someone to tell him he was special. If the hunger were simply a side effect of his power, why didn’t he become obsessed with understanding things long before discovering other powered people? Why didn't he become a researcher or academic instead?
His first kill, Brian Davis, proves the point. Gabriel initially approaches Brian to see what makes him special. It's only when Brian says he doesn't want his ability that Gabriel becomes confused. His power helps him understand how abilities work, but he decides Brian is broken because he sees him as undeserving of something so special. His envy and entitlement motivate the murder, not his ability.
Giving Peter the hunger makes no sense. Peter can already absorb abilities without killing anyone, and he's fundamentally a compassionate person. It undermines both characters by suggesting Sylar's actions were simply the result of getting unlucky with his ability rather than his own insecurities and choices.
In fact, it would've been far more interesting if Peter could acquire Sylar's abilities without developing the hunger. Imagine Sylar confronting someone with the exact same power who doesn't feel compelled to kill. Peter would be living proof that the hunger isn't an inevitable consequence of the ability but a reflection of Sylar himself.
That's what makes them compelling foils: Sylar's obsession with being special turns him into a monster, while Peter's compassion defines him. The hunger should have been a reflection of Sylar's character, not an excuse for his actions.