What is a bad relay? 

A bad relay is one that either doesn't work properly or tampers with our users' connections with maliciousness or misconfiguration
tampering with exit traffic in any way (including dropping accepted connections)
running HSDirs that harvest and probe .onion addresses
Report bad relays at bad-relays@lists.torproject.org with the relay's IP address or fingerprint, what kind of behavior you see, and any additional information needed to reproduce the issue.
Bad relays criteria: 
https://gitlab.torproject.org/tpo/network-health/
team/-/wikis/Criteria-for-rejecting-bad-relays
Blog post: Malicious relays and the health of the Tor network
https://blog.torproject.org/malicious-relays-health-tor-network/