I think that at the moment that he threatened them with the bottle, he was turning hard inside. He was becoming more like the rest of the gang -- liking violence more than he used to.
To me, this is an important point in the story. Pony has been the one who tries to "stay gold," who is not so violent as most of the Greasers. But at this point he seems to be changing.
But then Two-bit says something to him about not getting tough like the rest of them. Then Pony starts to pick up the broken glass. To me, this is a sign that he really is going to "stay gold" and not get all calloused like most of the Greasers.
In Chapter 12, Ponyboy says that "I didn't feel anything--- scared, mad, or anything. Just zero." After Johnny dies, he's in a state of numbness that is a result of guilt and depression. When the Socs jump him, Ponyboy breaks his bottle of Pepsi and holds the neck towards the Socs. The Socs immediately drive away, and Two-Bit is surprised by Ponyboy's actions. Previously, Ponyboy would never have threatened a Soc with a bottle. For example, in Chapter 3, he tells Cherry, referring to a broken bottle, "I couldn't use this...I couldn't ever cut anyone...."
However, depression has changed Ponyboy, who thinks to himself in Chapter 12, "I knew as well as he did that if you got tough you didn't get hurt. Get smart and nothing can touch you..." Ponyboy is trying to act tough as a way to protect himself from the loss and hurt he has experienced, but his friends know that Ponyboy isn't really violent. After this incident, for example, he begins to gather up the pieces of glass so no one gets a flat tire. This is the real Ponyboy--one who isn't violent. He is acting this way because he is depressed.