At the present, Severus hung back until he was certain the runes would work. He had no desire to be anywhere near the village until all was positive this was going to be successful. Severus was not a coward. Simply put: he had no desire to get captured once more. This time, he had a feeling he wouldn't be nearly so lucky should he be able to escape.
Severus' wand was at the ready even as he was a fair distance away. If they were lucky, this would work.