Technically, all skin cancers are dangerous if you don't treat them! Once you have had a single skin cancer, it is important for you to be evaluated at least every 6 months by a dermatologist. People who have one are more prone to develop other skin cancers, and if you've had more than one, that makes it even more likely that you will develop more over your lifetime. SCCs are dangerous if left untreated, but if you are seeing a dermatologist often and having suspicious sites evaluated and monitored, that is the best thing you can do.
"This answer has been solicited without seeing this patient and cannot be held as true medical advice, but only opinion. Seek in-person treatment with a trained medical professional for appropriate care."