Newt had a lot going on near the end of the third movie. His case, which he thought was the real one, disintegrated in his hands. So he thought all his creatures, including the newborn qilin, were killed. Before, when he had been in the Room of Requirement, he had taken what he had thought was the real case because he had thought he was the best one to protect the qilin and the rest of his creatures, but he ended up failing. There might have even been a part of him that felt like HE killed the qilin and the rest of his creatures because the case only disintegrated when HE took it. It was fine when it was in the acolyte's hands.
And then right after that, the animated dead qilin bowed to Grindelwald, so Grindelwald became the leader. So the team failed in their mission, and a murderous fanatic was going to be in control. And then Grindelwald gave his cruel speech and followed that up with torturing Jacob, Newt's best friend and a muggle (meaning that Jacob could not defend himself with magic) with the most painful kind of torture curse, a curse so evil it gets you a one-way ticket to Azkaban. Newt tried to intervene but was unable to because he was being held back by dark wizards. So he had to watch Jacob get tortured and be unable to do anything to help him.
All this happened right after another. No break in between. That's a lot!
And he still tried to help the Wizarding community by saying that Grindelwald didn't want to lead them and then talking about the second qilin, but he was struggling to because he was overcome by emotion and, with the qilin being dead, nothing could really be done to fix the election situation.
In the second movie, Theseus went through the most at the end. Newt went through a lot, too, but he didn't know most of the people who got killed. Theseus did, was responsible for them, and considered them family. In the third movie, Newt was the one who went through the most at the end.
Thankfully, most of it ended up working out though: It turned out Newt never had the real case, so the qilin and the rest of his creatures were alive, and a different candidate was chosen. Grindelwald did escape, but at least he wasn't the leader, and they could try to find him later.
