I think some posters are missing the fact that the OP's DP has watched a relative die, and refused to take her DD ("a little girl") as it would in his opinion "scar her for life."
He acted poorly but it may well have come from a good place.
Until you've watched a loved one die, you don't know how traumatic it is. I cannot tell you how much therapy I've had to try to come to terms with it.
He may well have been trying to protect the OP's DD, deciding that because of her young age, she wouldn't have been able to handle it. He had personal experience of watching someone die, he knew she was struggling with her mental health already and that he was one of the few safe people in her life, so he may well have felt the best person placed to make that decision. I'm not saying he made the right call or that it was his call to make, but I suspect his own grief might have been talking.
It doesn't matter how long ago his loss was, that kind of grief stays with you forever.
When he dropped and ran, he could just have been frustrated at the OP and her DD not trusting his counsel. He could have run away because of the memories that it brought up.
With hindsight, I think the OP should have discussed what was going to happen when the call came. He may well have expressed then that he was against it, giving the OP time to discuss it and come to an agreement, or to know she needed to plan to get a taxi.
Hindsight is always great.
I'm not sure DD will forgive him anytime soon - I wouldn't expect her to, she's dealing with so much without sparing any headspace for her mum's boyfriend - but that's not the main point here.
OP, you need to decide if your DP's motivations were coming from a good place or not. I think you need a very big conversation, and when you have it depends on how much you are grieving right now. DD's pain will be immense - you didn't lose your partner, but you did lose the father of your child, so you may well be hurting too much to have this conversation. Or, you may have come to terms with it already. It's hard to predict.
But at some point, you need to chat calmly and rationally with your DP to understand whether he reacted out of his own trauma with good intentions, or if he was just fed up of helping you and your DD and chose the worst possible moment to make his point.
Life isn't clear cut in black and white. He might not be the monster so many posters are making him out to be. He could just be someone who cares about you and your DD and has experienced a trauma he isn't actually over.