Exactly what I thought when I watched it!
In defense of the Chief and all of the other investigators in this case -
First, we don’t know what his “normal” face nay be. What we may perceive as smirks may just be his face.
But more importantly, these investigators have lived and breathed this case every moment of their lives since the moment they were called to the scene. They have lived this while under tremendous scrutiny and criticism - and, I’m sure, the fear that the killer could strike again before they could make an arrest. Not to mention that they are PEOPLE - fathers, mothers, siblings, etc - and these horrible murders will forever leave scars on their souls.
Knowing now that an arrest has been made, and as we await the press conference for more details, I can fully understand that these investigators feel at least a semblance of positivity/happiness about this and I don’t begrudge them one bit for this.
I actually feel for them and worry about them in the days, weeks, months and years that follow as the drive and dedication in their hunt for a killer comes to an end. This is when the full weight of the tragic deaths of Ethan, Kaylee, Maddie and Xana - that they had to compartmentalize away in order to focus on their search for the killer - will come crashing down on them.