As you all know, I believe Herold was assigned to be the assassin of Johnson. Atzerodt was not going to do it and Booth knew it so he gave the job to Herold. Herold, from what I've read, was a young man of lower than average intelligence who hero worshiped Booth. He could have been talked into it by Booth far easier than Atzerodt.
At this point we need to make clear what was planned and what occurred as a result of circumstances.
Anything that happened before 10pm was planned. After that it was every man for himself.
If you look at what was planned you can readily see that Atzerodt was going to be sacrificed when Herold purposely left Booth's jacket containing the map southward in his room. I believe that Booth meant to lure the posse southward by throwing enough red meat to the wolves following him. He mentioned the southward route to the guard at the bridge and even had Mary Surratt leave his field glasses along the trail. Meanwhile, he would turn northward and use the train system to escape to Canada.
When Herold was unable to kill Johnson because he stupidly left his gun in Atzerodt' s locked room, he took off after Booth like a lost dog after his master. Booth never planned on traveling with Herold because one man can blend easier into a crowd than 2 or 3. Booth's broken leg was his unforeseen circumstance that allowed Herold to catch up with him and help him get treatment.
Thats how I see it. Booth didn't give a rip about anybody but himself and his own escape. Mary Surratt, Lewis Powell, Davy Herold and George Atzerodt were all to be sacrificed....and they were.
BTW - Does anyone know whatever happened to Booth's field glasses?
That's a really interesting and intriguing point.