An addiction he had with no withdrawl or side effects (besides scarring, bruising, and bloodied clothes). An addiction that was his job, an addiction similar to the adrenaline junkie he was sometimes told he was.
Danger.
Danger was another addiction on a long list. The prospect of getting in trouble, running away without getting caught. The thrill of going against more men than one man should be able to take down. The blood pumping and sometimes gushing out during a fight. It was all in a heavenly killer package, a three for one deal.
His every day life at work was exactly this, and he adored it so. Fulfilling the ache in his chest after weeks without a job, the warm reminder of blood to tell him he was human and could die - gods, he loved it all.

He was addicted to danger, yes. On top of that, being a magnet for such an exciting and fun time (to him) was better than whiskey, better than nicotine (which isn't on his list). As much stress as keeping on his toes bought, Sebastian loved it all. The weight of guns and knives in his pockets, the countless ways he coils use both when the need arrives...
He adored it. Loved it. Craved it.

His reckless way of doing things made danger appear easier, something that was both a reward and curse, though the two often intertwine. Perhaps that's why excessively reckless actions were take.