9. Convinced Gary Lester To Accept A Demon - Hellblazer #2
If you know John Constantine on a personal level, then good news, I have an explanation for that odd ticking sound you hear every now and then! That's the clock ticking down on when John is gonna come into your life and royally screw you over in ways you didn't even think were possible.
For proof of this, look no further than poor, poor Gary Lester. In the second issue of the original Hellblazer run - because this is kind of a defining thing for Constantine so best to establish it asap - Gary enlisted the aid of his childhood friend to help him deal with a demon named Mnenmoth, The Lord Of Flies.
But the thing about demons in Constantine is that, unless you jump through very specific hoops, a mortal being cannot truly kill a demon. But you can trap it forever in a willing host. So Constantine, in one of the most morally abhorrent things he's done to this day (and hoo boy is that saying something) uses Gary's idolization of him to convince him into taking Mnenmoth into himself. This, naturally, absolutely destroys Lester's body from the inside out, with Constantine leaving him for dead in an insane asylum.
What a jerk.