You were in the wrong. The security guard was in the wrong. You have correctly agreed to apologise. The security guard should volunteer an apology but this has been made more difficult by you asking for one and appearing (?) to make it a condition of your own apology. His colleagues are supporting him because he has a lot more to think about right now than whether his pride will allow him to apologise to (what he probably sees as) an arrogant brat who made his life harder while he was trying to do his job on a day that had already turned out to be pretty awful.
Seeing a young person dying/dead is horrible. Seeing a sudden, unexpected, and traumatic death is horrible. Having responsibility (even if briefly before emergency services turn up) for such a scene is horrible, as is being put in that position for the first time. He is likely to take some leave to sort this out in his mind and will probably be profoundly effected by this for the rest of his life.
I personally would send him a genuine apology without any reservations or conditions. If you receive one in return then that's excellent and the world is (almost) right again. If you don't, you could complain about his conduct but I think you'd be an d*ck to do so.