Hi agarriberri –
A character’s motivation in my narratives is multilayered. I usually have one motivation that unifies all the others together.
It seems that you have chosen “moving on” from incident “x” as your overall arching motivation.
Let’s work with that.
You identified “overcoming trauma” as a layer of moving on… so how would your MC overcome the trauma of incident x?
Typically, this is where choice would come into play.
Some of your readers may desire to have the MC save a particular npc … perhaps one of those involved in the current murders has something in common with someone involved with incident x?
Other readers may wish to concentrate in having the npc solve the murders. What happened in incident x is prevented from happening once again, and by preventing history from repeating itself the MC is allowed to move on from the past.
Finally, maybe your readers love angst and pain, so some of them may never want the MC to overcome their trauma. Does this mean, they fail to solve the current murders? Or perhaps they fail to save the one innocent person involved in the new murders? And what happens when they can’t move on? Does the MC become a broken drunk? Or maybe something else?
Also – using cliches or tropes is sometimes ok … as long as you execute using them well.