So why couldn't you just tie Him to the cross?
Simple pragmatics. He could be untied and escape. This way he can't escape so easily.
And He's in a position to escape?
He's got followers.
And the fact that He's exhausted before you even nail Him to the cross doesn't affect your decision?
No. It's standard practice.
Hasn't He suffered enough?
More than my job's worth not to do it. Just following orders, y'know.