Well, since he was the very victim of that notion, Jem was condemning it. Regardless, Jem thought it was reckless for Miles to run ahead of them.
When they arrived, the situation in the area was suspicious. Just based on that, you will immediately realize that the environment is dangerous. But, Miles didn't seem to mind.
Jem thinks the situation is pretty familiar. It was the same when he joined Desty's group. Due to the reckless move of one of them, their lives were put in jeopardy. And that was also the beginning of all his painful calvary.
He was lying if he said that it never entered his mind to just leave Miles and the old man. And just save himself first. But, he still had a strong disgust at that idea. He didn't want to be like those people. That will use other people as tools to save oneself.
