Most of the people I spoke to saw themselves as providing a service - they wanted to help do something to bring a particular kind of future Internet about and found it rewarding to be a part of that. A number of them found the act of running a relay interesting and fun in itself - something they could get better at. Plus, membership of the relay community itself (especially now) is a kind of shared experience of community - and that's attractive to people in itself.

In terms of malicious actors, Tor does a lot to avoid this, from hunting down bad relays actively, monitoring the network as best as it can, continuously developing the algorithms which select routes through the network, and other mechanisms, like forcing relays to operate for a while before they get trusted with a lot of connections.