The scream from Betty for Hulk to stop served more than one purpose...
1) It showed the humanity within the Hulk (as Bruce), and control that Bruce was able to have... Hulk wasn't a mindless brute fighting another brute... It was Bruce, trying to "aim" the Hulk with a "purpose"... to stop Abom and his carnage (I don't think that Bruce thought to himself in the helicopter, when he appealed to Ross to let him face Abom... 'let me go down there so that I can kill this beast'). So this was a device to foreshadow how Banner was there at a conscious level, capable of control...
2) It also served as a display, in the difference between Dr. Bruce Banner and Emil Blonsky. A way to identify a hero (it is not just power, it is what motivates the hero as well).
Blonsky was a killer... Sure he was in the military, but it was for the fight, and sense of superiority and dominance. He wanted more serum after the Hulk beat him. He told Ross that he wanted another crack at Banner... He threatened to kill Stearns if he didn't give him the "god-like" powers that Banner had. Contrast to Banner, who wanted to get rid of the Hulk. Then, once Blonsky had the power, being a government sanctioned killer (special ops) didn't matter anymore... he was ready to trash everyone (even Ross). Hulk killed some soldiers, but it was when he was attacked, and in the heat of battle... Conversely, Abom waded through down-town Harlem looking to inflict mayhem and casulties... He chased the soldiers in the damaged HUMV so that he could crush them (ruthlessly killing a taxi driver in the process)... He threw a soldier (a fellow soldier, no less) out the window of the third story lab during the scene when he first emerged in that alley, after his transformation... He kicked some poor bastard into the air for no reason, after he hears the Hulk roar (after Hulk climbed out of the hole in the street).
So, in the final battle, we have a maddenned Hulk with head back and eyes closed, crazed with rage, ready to shear Aboms head clean off... The message is, Blonsky would done that even if totally lucid... Dr. Banner, a man who dedicated his life to science and discovery, who was trying to find a way to save soldiers' lives when he conducted the bio-tech-force-enhancement research in the first place, would not savagely strangle or decapitate someone.
So when Betty yelled 'stop'... we saw control... we saw Bruce. He had Abom defeated... the threat was neutralized... killing him woould have been excess. Banner would not do that... He willingly underwent a tranformation to save people, not to kill. A monsterous Hulk, would, Bruce would not. That is the point.