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 thenthan 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.