MySQL: Safe to kill transaction that is stuck in "Waiting for table metadata lock"
Yes, it's safe. Your alter table query hasn't even started.
However, the waiting for table metadata lock means, that there's another transaction operating on that table for a long time and you should rather kill that one to solve the problem. (read more about it here: 8.11.4 Metadata Locking)

To find this transaction, this thread is probably helpful: How do I find which transaction is causing a “Waiting for table metadata lock” state?