I've been watching the UBW series. When Caster abducts Shirou to take his Command Seals so she can use Saber against Berserker, Archer comes in and saves him - then fights Caster.
At some point during the fight, Caster sees Shirou unprotected, so she fires at him (at which point Archer saves him again).
Why did Caster shoot Shirou, if she hadn't retrieved his Command Seals yet?
It's not like Shirou was any sort of threat. Surely dealing with Archer first would be a better idea, then retrieve the Command Seals off Shirou (since he's no match against Caster) to be able to fight Berserker using Saber.
Caster was definitely aiming to kill - other than the fact that her attacks literally blow up the ground, she was also smiling (as in, "gotcha!"), and then was surprised to see Shirou still alive.