Watching a death occur in front of your eyes can definitely cause PTSD which is Post Traumatic Stress Disorder. That can include reliving the moment in your head (aka flashbacks) intrusive thoughts. Things around the victim might be triggers and cause overwhelming anxiety attacks as well. Depressive episodes are likely. With triggers also, feeling unsafe when you actually are occurs with someone suffering from grief. Nightmares of reliving the event, dreams of the person who died saying hurtful things they have never before and would never. They might start to tell themselves that they are the cause for the death.
Or, their body might numb itself, forget the event. They will still grieve but it might be short and their brain will push it mentally aside as a coping mechanism, possibly forgetting and losing track of their actual experiences, their own life, their memories might turn grey. I suggest just researching to be honest and watch or read blogs/articles about trauma from actual professionals, it’s more valuable and more reliable because they can tell you more than anyone here. (I don’t think there is an actual professional here that’s majored in psychology and this type of stuff.)