In my opinion the film is personal and should be returned undeveloped to the seller. If they wish to discard it, it's up to them. As for developing it myself....never.
I find it ironic on a site devoted to photography, and it's value, that people would be so cavalier as to reply with comments like, "you bought it, it's yours" and " I'd just throw it away".
If you bought a used car from someone and they accidentally left personal items in the trunk, would you say the same thing.
It says a lot about you.