Many of these people are probably ashamed of themselves now because they have deleted traces of their responses. Some went so far as to say the world is better off without "idiots" like Zeng-and they said this after authorities confirmed his death.
Yes, Zeng's fixation on the woman he had never met and had known through social media for just a week was senseless, but at his age it is understandable. You can say it is not really love and it is irrational. But most people at this age go through periods of immaturity and may suffer from bouts of helplessness that could end in tragedy. Unlike Zeng, most get through it without harming themselves or others.
Some accused Zeng of emotionally blackmailing the woman who wanted to jilt him. This seems partly true. By broadcasting his suicide on social media, it was not only a direct appeal to her, but an indirect appeal to others in a conscious or subconscious effort to pressure her. However, one thing is clear: Zeng was not totally committed to killing himself. He was not one of those who would die regardless of how much help is offered. He wanted to live. Had he withstood this, he might be well on his way to other amorous affairs and even laugh at his own sophomorically romantic sentiments, though he did not cite Goethe's young Werther as a precedent.
Did the jeers put him between a rock and a hard place so he felt he had no choice but to carry his threat through? Without further evidence it is hard to reach such a conclusion. Some of the jeering crowd probably did not mean ill because words of empathy were expected and harsh words like theirs might shock him back to his senses. It may work in some circumstances and with people of certain dispositions.