People seem to think Johnny's death was random and unnecessary, with it being simply to build Trevor's image, but that's not the case.
Throughout TLAD, you can see Johnny is hopelessly in love with Ashley. He would say that he didn't love her anymore, but his words didn't mean anything, since he was only trying to convince himself. As Ashley fantasized, they ended up riding off into the sunset (the West Coast, California) and leaving everything behind, because there was nothing left.
The Lost was Johnny's purpose, but with it gone, he became an empty, broken spirit... easily susceptible to turning to drugs, especially with his company being a meth addict.
And so Johnny's end wasn't unnecessary, it was a realistic conclusion indeed.
Niko had gotten into a six star chase in Liberty tonight. With only a tiny amount of armour left, which I gained from a NOOSE van, and an inch worth of health, I was desparetly running through alleys in North Holland, searching for a car to escape the mass of police onfoot trailing me.
I saw a nice Virgo. Before getting into it, I grabbed a hotdog. Niko's clothes (he was wearing his original outfit btw) weren't bloodstained anymore.
They didn't spot me jacking the car, and so I passed by, making it to the Pay 'N' Spray.
It was now early morning. I drove across the bridge to Bohan. Parking underneath the subway, I jogged up the stairs. The train had just pulled in, and I entered it. It left into the morning sun.
I had to turn off the game right there, it was too fitting. That's the last time I'm playing GTAIV, as in, the last time before playing GTAV - we all know that after playing the latest GTA, the previous one isn't the same anymore, only playable for nostalgia's sake.
Thanks Rockstar for Grand Theft Auto IV!
Bonus: Here's a track from the game that always brings me back to when it was first released: