Aside from the death in the present day timeline, this was my biggest complaint.
These writers expect us to believe that a seasoned detective is going to waltz into a commune operated by a known cult and accept a concocted drink from one of their members? Unbelievably stupid and convenient writing.
You're critiquing based on the assumption that the characters knew what the audience did. They just followed Jeff and Callie up there, they had no idea what was going on and didn't have a reason to think that anyone on the compound aside from Shauna was a threat.
I have a much bigger problem with what happened to mustache cop than what happened to murdered cop.
Everything was wrapped up too neatly in that finale and there is no reason to think that there is a hero left in either the modern storyline or the past one.