Making him low enough level for an evil party to kill in combat isn't good for the credibility of the game world. It's as lazy as level scaling wolves up to 10 so that they still offer a challenge for a 12th level party.
If I was DMing Halsin would be much higher in level like an archdruid of a circle would be, at least 11+. The evil party would have to play dirty, cheat and poison him or something before having a chance in a fair fight. Or they would have to receive help from an evil faction who also wants the Circle gone.
Point is, there are narrative ways to keep things consistent and balance the level differences. "Halsin's revenge" isn't great writing as it is.
A powerful Halsin would be inconsistent with the narative, because if he were that powerful he'd a) go alone to the goblin camp, no need for some hired muscle, and b) kill Minthara & goblins on his own. Whether it makes sense for him to be weaker than expected in combat (which the other druids don't seem surprised by), depends on the full story behind the grove and the battle with the Sharrans.
5e does a good job of letting a swarm of lower level creatures whittle down higher level characters.
Maybe there were more powerful enemies present when Halsin attacked the fort. Maybe Priestess Gut landed a Hold Person on him and he just got destroyed.
High level in 5e doesn't mean untouchable like in DnD 3.x and again, he could have easily met and got captured by higher level foes that left before Tav went there.
A much more believable scenario than a level 5 "archdruid" leading a circle who got murdered by a level 3-4 party.
