If I read this right, you broke a locknut and now are worked up about it.
Don't sweat it.
Yes, these things happen. Low end locknuts are case hardened so the serrations are harder than the dropouts and they can bite and hold against sliding forces on horizontal dropout. As such they can be somewhat brittle and will crack from time to time. It's not a safety issue since they don't crack while the bike is in use, and even if they did, the worst that might happen is the wheel sliding a bit.
Go to a bike co-op or an old time "dirt under the fingernails" bike shop and they'll dig up a locknut out of the scrap hub bin.
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