Some inequality in wealth is ok and healthy: it allows to concentrate capital and build big things that a co-op usually cannot build.
Extreme inequality is dangerous because the few wealthiest people concentrate enough power to overpower the mechanisms that keep the society stable and working. If you can bribe your way out of problems with law, or force the hand of the government by lobbying or menacing, you can break the society, while also acquiring even more wealth. If not stopped, the process leads to a dictatorship at the mildest, or a downfall at the most extreme.
Extreme inequality is dangerous because the few wealthiest people concentrate enough power to overpower the mechanisms that keep the society stable and working. If you can bribe your way out of problems with law, or force the hand of the government by lobbying or menacing, you can break the society, while also acquiring even more wealth. If not stopped, the process leads to a dictatorship at the mildest, or a downfall at the most extreme.