A tenant illegally locking their landlord out and causing delay of several hours for a time-sensitive emergency would almost assuredly get a lot more % liability blame than a tenant who properly gave their landlord a copy of the key and who was able to give access to the plumber immediately.
Hopefully they have enough renters insurance to cover the claim and their insurance company will handle the court side of things when other tenants sue!
That's not the question at hand at all though. The question is what kind of liability you incur by legally locking your landlord out.
IANAL, but I'd guess that if you change the locks in a state where it's not legal to do so you're totally hosed in terms of liability. I have no idea how liable and to what degree renter's insurance would cover you if you change the locks in a state that guarantees your right to do so.
I've rented an over-the-garage studio in the past, and the landlord came in one time without notice when I was away because the supply line to the toilet burst and there was water dripping from the garage ceiling. That's the sort of emergency we're talking about here, where a landlord needs immediate access, and any delay will cause additional structural damage.
This is one of those cases where owners have rights too, and renters who want more rights should consider owning.