Death is a reality where there is much in way of struggle to understand. Both expected and unexpected death brings with it a series of questions and points to which answers are difficult to derive. In either situation, little in way of absolute totality can be reached.
One significant difference between grieving an anticipated loss versus an unexpected one due to suicide are the types of questions that follow each loss. Grieving the death of a loved one that was anticipated enables the grieving process to have some frame of reference to it. For example, when a person is diagnosed with a terminal case of a disease, it enables loved ones to prepare, to a certain extent, for the impending loss. Time is present to deal with the difficult reality that loss brings. Yet, this experience is not necessarily present when examining loss that accompanies a suicide. In these settings, there is a shock associated with the loss. Questions that accompany this grieving process such as how such an event could happen and simply grappling with its condition. For example, it is not unusual for grieving the loss of one lost to suicide to focus on how "yesterday morning, _____ was here and now they're gone." This helps to differentiate the grieving process between both realities.
As the questions that accompany each might be different, so, too, would the grieving process. In the process of an anticipated loss, the grieving focuses on the issue of death and loss. These are universal conditions that impact all individuals and help to broaden the grieving process. What ends up accompanying the loss of a loved one due to suicide embraces this universal condition of loss. Yet, it also focuses on the inevitable question of "why." The loss of an individual to suicide causes loved ones who survive them to ask questions such as whether or not they could have noticed the signs of depression or whether they did enough to help the individual. When confronting the reality of an anticipated loss to something like a disease, these questions are not as pointed on the individual. The grieving process for one who is lost to suicide embraces a more personalized and subjective approach in conjunction with a universalized approach towards death and loss. This would be another set of differences between grieving an anticipated loss of an individual verses the loss of a loved one to suicide.
Speaking as someone who has experienced both forms of loss, I can say that the major differences include feelings of shock and betrayal. There is almost always an element of shock surrounding the death of a loved one, unless of course there is a long battle with a fatal disease in which case it is almost always expected o happen at any time. But even in the event that the loved one is an elderly person, it isn't quite as much of a shock because we learn from an early age that people die in old age. It is still a surprise when it happens and we still grieve the loss, but it is the natural order of things and so we accept it.
With suicide, however, we feel left behind. We see the selfishness of suicide as a personal attack...why would you leave me? Why would you forsake our relationship? Knowing what this would do to me, how could you do it? Why didn't you warn me? Why didn't you ask for help? How could you leave your parents/children/friends? The grief is amplified by anger, betrayal, shock, and resentment. It might take longer to recover from the grief associated with suicide, because we harbor those additional feelings.