Apart from that I agree with fylimar, being captured and then staying in the dungeon as target practice isn't much of a plan - and it never was the plan. He leaves the Grove in the worst possible moment to follow Aradin and an old fancy of his (which he didn't seem to have done anything about during the last century), getting captured or gathering intel wasn't the motivation, he went with Aradin to learn about the Nightsong and and a way to lift the Shadowcurse. He tells you as much during one of your conversations after freeing him, the morning after the party iirc.
Plans change.
Seemingly, after finding out about the goblin army and its leaders, Halsin's plan changed to dealing with this and the threat it posed to the Grove at some point after leaving and by the time they were discovered.
Both Aradin and Liam will tell you he ran off on his own at the point where they were ambushed. Meaning, Halsin's motivation was not to escape at that point. Otherwise he would have stood his ground and helped Adarin's group escape.
Thus one can conclude that Halsin's idea was twofold: 1) Be a distraction for Aradin's group, by turning into a bear and running deeper into temple. Prompting the goblins to focus on him. 2) To somehow deal with the goblin leaders that were inciting the goblins to attack locals and as such were threatening the Grove (It's likely he and Aradin's group had spotted Priestess Gut and heard about Minthara and Ragzlin)
Being captured effectively accoplishes part 1. He was a distraction, enough that at least 2 of the 3 members of Aradin's group got away. Meanwhile, he is kept relatively close to the goblin leaders within the camp allowing him to potentially accomplish part 2 (In which he succeeds when Minthara attacks, when he shows up post-attack he has apparently killed Ragzlin and Gut - Though I don't recall if I ever actually checked in a run where I decided to do the Grove attack)
