I have been doing this job for around 8 years, and I find out that there are very big percent of foreign teachers in China just garbage.

The garbage FT's in China you're on about tend to be a transient group. Most of 'em can't adjust to working and living in China, so they leave the country for good, quickly or even after a few years in some cases. They continue their teaching careers elsewhere. But unfortunately, there are a few, mainly old goats, who have had such a bad time in China that they cannot get over it. That's why they post continually saying bad things about the country. In fact, they have become China bashers!