Grief is a complicated emotion. It isn't just feelings of sadness and loss. It involves the mind's ability to process sadness and loss. Regret is one of the stages of the mind in the grief process. It can be regret for the choices of the deceased or regret for the choices of the suffering for all the things that should or could have been. And there is usually a good reason for what might have been that did not happen. What might have been could only have happened if things were different to begin with. We do not normally think "this person might die so I'd better . . ." because we are usually too busy dealing with life in the moment. Families who suffered the loss of loved ones in the 911 tragedy parted in the morning the way they did each morning. No one had a reason to think: I'd better do something different this morning.
This is a difficult part of the grieving process, that is, to wrap the mind around accepting what happened. But who wants to accept such enormous grief? In order to accept what happened means the grieving have to endure another loss: the loss of the chance they could have avoided their grief. Most times, that is clearly not so. When the suffering are caught in the thinking of the maybe ifs, the experience can feel irrational and crazy. Grief in general feels irrational and crazy. Emotions take over and the mind races to make sense of it all. Ironically, the regretting stage of struggling with acceptance of what actually happened is quite rational given the nature of the grieving process.
There will always be the what ifs when we loose someone. Remember: if matters could have been different they would have been different. Acceptance is one of the stages of Kublor-Ross's five stages of grief. It will happen to anyone grieving. Continue through the process; it is the only way to heal the depth of grief.