Why did this character want to wait?

A few minutes earlier Voldemort issued an ultimatum to the occupants of Hogwarts Castle, give up Harry Potter and face no attack.

VOLDEMORT (V.O.) I wish you no harm. I have great respect for the students of Hogwarts. I was once one myself after all. I ask for but one thing and if granted no magical blood shall be spilt...

[For a moment they stand, hung in silence...]

Give me Harry Potter. Do this and none shall be harmed. Give me Harry Potter and I shall leave Hogwarts untouched. Give me Harry Potter and you will be rewarded.

_{^{HARRY POTTER AND THE DEATHLY HALLOWS - PART 2: Original Screenplay}}

Pius Thicknesse (Voldemort's patsy) is merely pointing this out to Voldemort. Big V, on the other hand, is under no illusions that they're going to offer up Potter. On arrival he finds serried ranks of giant statues and a great big shield erected to defy him.

THICKNESSE: But, my Lord, shouldn’t we wait --

[Voldemort’s eyes shift, killing the remainder of Thicknesse’s sentence before it can escape his throat. Bellatrix’s eyes narrow in contempt. Chilled, Thicknesse looks away. Then -- as if burying a knife deep in the heart of an enemy... Bellatrix drops her arm.]

You'll note that later in the script when Voldemort no longer has any use for Thicknesse, he kills him without batting an eyelid. Clearly he has no time for cowards and nay-sayers.

VOLDEMORT: Nagini. Come.

[Voldemort begins to slip away, BLASTING anyone who stands in his way. Pius Thicknesse comes INTO VIEW. His eyes widen. Voldemort raises his wand and, with callous disregard, executes him as well. As those ahead part for him and Nagini, we return to...]