Toward the end of T2, a trucker checks on the T-1000 after it crashes in its helicopter. On its way to commandeering the truck, the terminator impales the man, unquestionably killing him.
I get that the T-1000 is the "bad guy," but what is the in-universe reason for this? The terminator doesn't always kill, even when it can. If anything, the effort to dispatch the trucker cost the T-1000 a few seconds.
Terminators are pretty ruthless when they're 'on-mission'. There's a reasonable chance that the lorry driver (Dana Shorte) would resist even a policeman taking his rig without permission, which could then waste valuable seconds when it comes to killing John Connor. Better to incapacitate him away from the lorry than have him grabbing at you from behind. The script and screenplay both note that the driver is killed without the T-1000 Terminator "even looking at him".
Out-of-universe, the major goal here is to humanise the T-800 Terminator in comparison to the deeply inhuman T-1000. Instead of killing the van driver to get his vehicle, it asks him nicely(ish) and waits for him to get out of the way.
“Goddamn, are you all ri—” Dana managed to get out before—
SSSSHHCK! The T-1000 drove a blade through his abdomen and walked on past without slowing, or even looking at him.
Dana sank to his knees, reaching for the hot furnace of pain that his stomach had just become. He could feel things loosen and shift as he slapped his hands over the gushing tear in his belly. Before it really hit him that he had been badly cut open, he watched, dumbfounded, as the policeman with the harpoon-hand calmly climbed into the open cab of his tanker, then released the brake. The huge vehicle bellowed and rolled forward, churning out exhaust like a berserk dragon.
The script is pretty much identical.
The shaken DRIVER jumps down.
The [sic] behind the wreckage a cop emerges, walking toward him.DRIVER
Goddamn, are you alri--SSSHHCK! T-1000 drives a blade through the man's abdomen and walks on past without slowing, or even looking at him.
The T-1000 wasted no more than two or three seconds impaling the trucker on its arm, and if it were in any great hurry, it logically should've been running towards the truck, rather than walking. Its behaviour suggests it wasn't in any great rush at that point.
As for what it gained by killing the trucker, perhaps it wanted to ensure that the trucker wouldn't have the chance to alert the authorities as to the theft of his truck. The T-1000 didn't have much to fear from the police in terms of its own safety, but coming under fire from the police while still pursuing its targets would likely have proved at least a minor hindrance. The scene is reminiscent of an earlier scene where The T-1000 either gut-punched or stabbed (likely the latter) a police officer before stealing his car, presumably for the same reason.
Edit: In addition to what I wrote above, I think Jenayah was correct in suggesting that the T-1000 had a rudimentary personality. Jenayah cited the T-1000 wagging its finger at Sarah Connor in the steel mill, which hints at a limited sense of humour. And after hurling the T-800 through a window during the fight in the mall, the T-1000 paused to look at a silver mannequin, frowning slightly, before continuing its pursuit of John.
Now I don't necessarily think it killed the truck driver out of irritation, since it dispatched that police officer in a similar fashion shortly after arriving in 1995, when it had no apparent reason to be annoyed. Seems more likely that it was just silencing a potential witness to its actions, in both cases. However, if the T-1000 did indeed possess a rudimentary personality of some sort, it follows that it wasn't wholly single-minded in its pursuit of its mission objectives. It had some degree of free will, and could make choices that didn't strictly benefit its mission.
I think this can reasonably be inferred from its actions within the movie itself, but the novelisation is more explicit:
"it was fully autonomous, and barely under the allegiance of its creator, SKYNET. SKYNET had hesitated before creating this latest weapon system. There were unpredictability factors related to the liquid poly-mimetic alloy's longevity and the ability to process commands without interpolating its own priorities over those of its creator. it was so volatile a construct that only in the last throes of defeat, only when it appeared that the Resistance would finally be able to mount an offensive against the inner command components of SKYNET, even threatening the Cheyenne Mountain complex itself, did SKYNET go ahead and create the T1000. Einstein once said that God didn't play dice with the universe. SKYNET had no choice..."
Sourced from: http://www.jamescamerononline.com/T2FAQ.htm
To me, it simply felt like the T-1000 was being pissed off from having been outsmarted (again), and lashed out in killing the trucker.
One could argue that Terminators are robots, supposed to have a cool head with no feelings and only focus on the mission, but this particular Terminator is able to be mocking and seemingly enjoying taking its time to kill (when he's about to poke Sarah's eyes with a spiky finger). Why wouldn't it be able to feel irritated as well?
Cold, hard machine logic.
The T-1000 has determined that the most optimal way to continue its mission is to commandeer the truck.
Based on the rules of the human society that the T-1000 has been inserted into, the human that has descended from the truck can reasonably be expected to have an attachment to the vehicle. Thus they will almost certainly view the T-1000's attempt to take the truck as theft, and actively oppose it.
The T-1000 has no idea what form this opposition could take. It's unlikely that said opposition would take long for the android to overcome, but it's quite possible that the driver pulls a gun on the android. While the T-1000 is of course impervious to bullets, the truck is not - the driver could inadvertently damage or disable the vehicle while attempting to stop the android from stealing it.
This is only one possible negative scenario that leaving the driver alive could result in. There are millions of others that the T-1000 could simulate with the data available to it, and millions more that it lacks data for ("unknown unknowns").
But why waste time computing scenarios and their consequences, when you can simply eliminate the primary variable in any of those scenarios, thus invalidating them all? No driver means no opposition. No opposition means the truck can be taken without trouble. Which means the T-1000 can continue its mission.
Thus, the T-1000 executes the truck driver because doing so is both the simplest and safest way to ensure its mission succeeds.
(Aside: the T-1000's method of execution is decidedly sub-optimal considering the theory of a logical machine outlined above. Ideally it would have killed the driver instantaneously, perhaps by decapitating him or breaking his neck, as this would guarantee the driver could not use the last few seconds of his life to possibly interfere with the mission. But that would've made the novel and screenplay less dramatic for the mass market, hence the unnecessary abdomen-stabbing and driver-bleeding-out.)
My answer is, why wouldn't it? The same scenario occurred near the end of the first film when the T-800 knocked out a truck driver who ran over him. We don't know whether the truck driver was killed or simply knocked out.
But why wouldn't the T-1000 kill the truck driver? I mean, what else should it have done? Ask politely?
The simple answer is, convenience. Killing the trucker would allow the T-1000 to easily take control of the truck and chase his victims.
