Who is Gabriel referring to when it told Constantine he took a life?

Question

In the building where Gabriel and Constantine spoke, he expressed his dismay that he has not repaid his debt to God to which Gabriel replies,

You are going to die young because you smoked 30 cigarettes a day since you were 15... and you're going to go to hell because of the life you took.

Who is Gabriel referring to? The only instance I could think of is Gabriel referring to Constantine himself when he was talking to the doctor

You saved me before, you can do it again, right?

Answer 1

Since the reason for Constantine's first visit to hell (and now his tries to buy himself into heaven by doing heaven's dirty work) was his suicide (attempt?) as a teenager, I' m pretty sure Gabriel is really just referring to Constantine himself, meaning that he took his own life (which for Christians is indeed a reason to go to hell).

Answer 2

The first life he took was his unborn twin brother he strangled with his own umbilical cord while in the womb. See the wikipedia article.