Kim-
Don't worry, it's natural to have those feelings after losing a family member in such a way. I also understand where you are coming from when you say you can't imagine why someone would commit suicide. You aren't promised tomorrow.
As for getting rid of these thoughts, have you tried to concentrait on something else, something happier?
Worst comes to worst, you can always see a psychiatrist. I know when I was having some strange ideas of my own, my therapist gave me some great ideas to manage. Including thinking happier thoughts.