As a former facilitator of a grief support group, I can tell you that the grief process doesn't work to a calendar. A lot of factors are at play here. What is the age of your MC? A teenager or 20-something who accidentally kills both of his parents would have what they call grief cycles. When a young person suffers a major loss, other major life changes--such as moving into adolescence or getting married--can resurrect the worst feelings of grief. Each growth period requires a new phase of adjustment. So, if your character was still in elementary school or jr. high, he may very well feel new grief two years later.
One way to think about this is in the category of firsts or milestones: The first Christmas without the parents; the first birthday without the parents; the parents' wedding anniversary; the day your MC gets his driver's license; the character's first date to the prom; the day he graduates from high school, etc. Each and every one of those moments could resurrect the grief.
An older adult may suffer more from survivor's guilt, depending on the circumstances of the deaths. An adult's grief tends to linger in anger for shorter periods and be more concentrated in sadness and depression.
It's also useful to remember that grief is more than just anger or acting out. It's also manifested as deep sadness, emotional paralysis (unable to make decisions), longing for the past, and ineffective communication--both verbally and in writing.
As for losing one's parents, my parents died a month and half apart from natural causes when I was in my 40s. It was definitely a shock. My mother's death wasn't unexpected (she had Alzheimer's disease), but my father's death was quite sudden. I was an only child, so I was left to fend with my grief, the rapid burial/cremation of two parents, and the settlement two estates. Fortunately, my husband was tremendously supportive. I don't think I would have gotten through it without him.
Hope that helps answer your questions a little.