After Robert tells Roger how he can prevent him from coming up the ladder in Chapter 10, how does Roger characterize Jack?

Here, Roger is returning to camp & meets Robert, the sentry. This is after Jack's faction has split from the rest of the boys. As Roger comes up the ladder, Robert forces him to identify himself before he can continue.

Roger peered up.
“You couldn’t stop me coming if I wanted.”
“Couldn’t I? Climb up and see.”
Roger clambered up the ladder-like cliff.
“Look at this.”
A log had been jammed under the topmost rock and another lever under that. Robert leaned lightly on the lever and the rock groaned. A full effort would send the rock thundering down to the neck of land. Roger admired.
“He’s a proper chief, isn’t he?”

So Jack has arranged their defenses, only allowing those who are "friendly" to his tribe to come up the ladder. this appeals to Roger's violent, dangerous nature. He is the one boy who may truly be described as sadistic, the one who actually enjoys inflicting pain on others. Of course, this idea of Jack's may have inspired Roger's choice of weapon in Piggy's death as well. It's clear that Roger admires Jack's mode of governing, & his controlling authority.