1 Hosts are programmed not to kill
The Hosts in Westworld are programmed not to kill guests. This was evident in many situations where a host actually tried to harm a guest but failed to do the right move.
2 Bullets have velocity safeguards
According to discoverwestworld.com, bullets have velocity safeguards. This means that the speed of bullets are reduced in such a way that they sting humans but never harm them.
3 The bodies of hosts are weaker
The bodies of hosts could have been designed in such a way that they are weaker than the bodies of humans. As a result, the low velocity bullets can easily penetrate the body of a host but not a guest. See also plot explanation of season 1.
4 The bullets detect body heat
According to some fan theories that are based on the 1973 movie, the bullets could have heat sensors in a such a way that they detect human bodies and stop before hitting them.
5 The guns are real
According to Jonathan Nolan, one of the creators of Westworld, the guns used are real guns but the bullets are like the ones the US army uses for training. Those bullets cause a sting but they never kill. See why some people don't like westworld.
6 Hosts are programmed to protect guests
Nolan said that a part of the hosts unconscious programming is to protect guests in case they put themselves in danger. Nolan said that if a drunk guest walks towards the edge of a cliff, a host is likely to help without breaking the narrative.
7 Guests can't kill other guests easily
Because the saftey mechanism lies within the bullets, guests can't kill other guests easily. However, this can still happen using any other killing method.
8 Guests can die of accidents
In Westworld, guests can die as a result of an accident that is usually not related to gunfire. A few guests died in Westworld as a result of different accidents, which is why guests have to sign a paper that they won't sue the park in case anything wrong happens.