Brokers do not earn spreads only. It depends on what kind of model the brokerage firm adopts. For market makers, in most cases, your loss is their gain unless they hedge off your positions in the market. Brokers also earn from rollover fees. At times, when you check out the buy and sell rollover fees, you will notice that both columns are negative.
You can lose more than your equity. Do you remember the Swiss currency crisis in Jan of 2015? Because of huge movement in Swiss pairs, clients lose more than what they have in their accounts, resulting in negative equity. When such cases happen, your broker either waives off or you are asked to pay for whatever you owe. In view of this, please make sure if your broker offers negative balance protection.