He puts his arm in front of the Serpent, showing that he wants them to stay put for a moment. Stalker thought to himself and realized that if they upfront rushed in and attacked in the open, they would only have more and more troops come to stop them. As natural of killers as Stalker and his uneasy ally were, they would definitely be overwhelmed. Sneaking in wouldn't alarm any troops, therefore would allow them to escape without being followed. But at the same time, sneaking would proof more than difficult now that some of the troops knew that there was something on the loose.