Why stab him?

Good question. I figured that they didn't need to get the answers out of Dave. My question would be (after killing Dave)is why they wouldn't call Dave's contact at the hotel. Fred had given Dave the number to call at the hotel to do some business. Once calling them, the buyers would tell Fred that an old man came to their place and sold the some of the stuff. Fred would quickly deduce that it was Lou who was there. Fred obviously knew Lou since he ran some errands/bets for him.

Last but not least, Fred had ratted out Dave to those thugs that chased him into mobile parking unit. Remember Fred was amongst them in the white limo.

"How does that get either your money or your coke back? " Unfortunately my answer to that above, brings on more questions. Maybe someone has a theory aside from the (we don't need the "you need to suspend your disbelief nonsense).

Anyways I hope that helps a little.

I'll take a 'stab' at it.

For starters, the filmmakers have to establish that the bad guy is going to have a knife to attack Burt at the end of the film. It can't be a gun, because then he'd just shoot Burt. And if he didn't have a weapon, he'd just run away because Burt had a gun. So, that's why he was carrying a knife in the first place.

As to why he stabbed him, I suspect that the original scene was he came at the kid, they might have struggled and, at some point, the kid would fall off the rotating parking garage to his death accidentally.

But, for some reason, they couldn't do the scene that way. Maybe they couldn't get permission to do it on that kind of garage (I've never seen one of those before or since in a movie) or the logistics (or budget) just wouldn't permit it. AC isn't exactly a big-budget action flick. For whatever reason, they just couldn't have the kid die in a way that would have made sense.

So, they decided to do some half-assed, stylish homage to The Godfather, Part II, when Robert DeNiro returns to Italy to kill the Don who killed his brother and mother. In that movie, DeNiro stabs the Don in about the same place as the kid in AC, but DeNiro also brings the knife across his stomach. That's what I was expecting the bad guy to do in AC but, instead, he just kind of holds it there. Again, maybe they didn't have the budget to set-up that kind of scene and all they could afford was one, single stab wound.

Anyway, that's my theory as to the reason for seemingly senseless stabbing of the kid before they even tried to find out where the drugs/money were.

I took that killing as an impetuous act. Sensibly, they should talk to him to get the dope and money back. But I guess they felt anger at the kid for making them run after him, and they felt further anger about his robbing them in Philadelphia. Their anger just overpowered their good judgment.

The scene helped convey how brutal and animalistic the bad guys were

meh.
just to get the rest of the film going