I think they had to be wary of police during the wandering period. I admittedly don't know shit, but that always seems left out, and that's my guess for the dominant emotion during the wandering period, not remorse. For one, Eric quoted "feel no remorse, no sense of shame" from "Anarchy" multiple times. I can't imagine they predicted the cops wouldn't rush in, but sometimes it seems like that's how the massacre is imagined.
In the library they were saying things like "cover me" and there were just kids in there. Contra depictions like say Zero Hour, I think there was a lot more watching your back in the halls. I don't have the knowledge of the lay out of the school to say it with confidence, but I think that might be why they were wandering. To find if or where the cops were, checking exits, etc, and assuming they had already entered. They wanted to kill cops, and the vision was not just shooting them from such a distance such that they never even hit one.
I also speculate that they still thought the bombs would go off and collapse the library during the library massacre. If that's true, they would have been a lot more confident about facing the cops while in the library - and in fact the kids in there state they entered with confidence, and a lot less while roaming the halls.