idiotic English only policy makes posting Polish language sources problematic.
You can post Polish sources, but you're supposed to provide a translation or summarize it as obviously not all members here read Polish.
So far there doesn't appear to be much about it in mainstream coverage, but I did find a reporting by rt.com which doesn't mention the theft, but does mention the firework.
No one deserves to be killed for petty theft...
If the reports are true, then throwing a firework into someone's business premises is not just theft. That could be classified as attempted arson.
No, he didn't deserve to be stabbed to death, it is a an awful outcome, but undoubtedly this will now fuel racial tensions even more. Would the ensuing riots have occurred had that business been a Polish run one? Probably not.