Yes, watching someone die can cause PTSD. PTSD, or post traumatic stress disorder, can develop after vicarious trauma. With vicarious trauma, you can experience and feel the trauma and painful experiences by just watching someone else go through it. So if you watch someone you love die and if you develop PTSD, then you continue to re-experience the trauma and all the distress and feelings associated with it even when your loved one has long since passed away.
When you are exposed to any life-threatening trauma, the risk of developing PTSD is 9.2%. When you are exposed to life-threatening violence, then your risk of developing PTSD is 20.9%, the highest risk among all the traumas. If you witness the sudden death of a loved one, then your risk of developing PTSD is moderate, with 14.3% eventually developing PTSD (Breslau et al., 1998). In this study, it did find that the most common trauma exposure was witnessing the unexpected death of a loved one, with 60% of the sample experiencing it. In addition, witnessing the sudden death of a loved one accounted for 31% of all PTSD cases (Breslau et al., 1998).
As studies illustrate, watching someone die can cause PTSD. And this is especially true if you watch someone you love die unexpectedly. This might be exacerbated by guilt feelings that come up about the deceased loved one, such as not being able to say goodbye, not being able to have closure with previous disagreements or conflicts, or not being able to tell your loved one how you really feel about them. So this inability to have closure may in itself be distressing and a source of much guilt and regret.
If you have watched someone die, it is important to talk with a mental health professional and also your support network (family, friends) to work it through and bolster your coping skills.