Wilhelm did plan to stand back and watch as August's friends were killed, but the instant they started fighting back changed his mind.
They were not giving up hope when faced with an unscalable wall. It annoyed him that they thought they could find a way out of this situation.
More importantly, if they stalled long enough and August came back, wasn't his surprise going to be ruined?
In order to slaughter them and leave their heads as a gift for the young Azure Dragon, he actually decided to move himself.
He knew just as well as they did that they wouldn't be able to fight him individually. On top of that, when he was backed by so many geniuses, he could torment them from a hidden position and take them down without a fight.
He was going to have fun.
That was his thought as he stepped forward to enter the fray.