Why doesn't Barris not care to save Luckman?

You have to look at this through a dark lens. Barris' action don't make a heck of a lot of sense because those actions are being guided by his addiction, paranoia and an unhealthy dose of mental illness. Within the film's source novel, although we see what happened in vivid detail, the motivations behind them really aren't any clearer.

• He initially ignores his friend as he obviously chokes, pretending to be transfixed by his work

Barris halted in his hash-pipe making, gazed up at hapless Luckman. In a frenzy, now gargling horrid noises, Luckman with one hand swept the coffee table bare of beer cans and food; everything clattered down. The cats sped off, terrified. Still, Barris sat gazing fixedly at him. Luckman lurched a few steps toward the kitchen; the scanner there, on its cube before Fred’s horrified eyes, picked up Luckman as he groped blindly in the kitchen semidarkness for a glass, tried to turn on the faucet and fill it with water. At the monitor, Fred jumped up; transfixed, on Monitor Two he saw Barris, still seated, return to painstakingly winding string around and around the bowl of his hash pipe. Barris did not look up again; Monitor Two showed him again intently at work.

• He is spurred into action for evident good reason. Fred thinks that he might be prepping his excuse for after the ambulance arrives.

For several minutes Luckman lay on the kitchen floor without moving as Barris wound and wound the string, Barris bent over like an intent old lady knitting, smiling to himself, smiling on and on, and rocking a trifle; then abruptly Barris tossed the hash pipe away, stood up, gazed acutely at Luckman’s form on the kitchen floor, the broken water glass beside him, all the debris and pans and broken plates, and then Barris’s face suddenly reacted with mock dismay. Barris tore off his shades, his eyes widened grotesquely, he flapped his arms in helpless fright, he ran about a little here and there, then scuttled toward Luckman, paused a few feet from him, ran back, panting now.

He’s building up his act, Fred realized. He’s getting his panic-and-discovery act together. Like he just came onto the scene. Barris, on the cube of Monitor Two, twisted about, gasped in grief, his face dark red, and then he hobbled to the phone, yanked it up, dropped it, picked it up with trembling fingers … he has just discovered that Luckman, alone in the kitchen, has choked to death on a piece of food, Fred realized; with no one there to hear him or help him. And now Barris is frantically trying to summon help. Too late.

• Ultimately, Barris' actions are inscrutable, as you might expect

Watching all this, Fred felt the fear drain from him. The man would be okay. But Barris! What sort of person was he? Luckman had recovered despite him. What a freak, he thought. What a kinky freak. Where’s his head at, just to stand idle like that?