Yes, it was, and we had to correct that flaw in our principles at considerable expense..
Did Americans who aided slaves in escaping to Canada violate the Constitution?
Depends on how the fugitive slave law was written, doesn't it? But sure, if that law was written properly, they probably were guilty of violating the slave owners property rights.
If I'd have been on a jury trying such a case, I'd have voted for a stiff fine.. Say a dollar a head.