Think about dangerous owners you have met. There are so many labels/signs about their behaviour that warn me loud and clear, they are dangerous.
here's some ive noticed...
they walk across fields, not around the perimeter so as to not ruin crops.
they leave gates open
they pick their dogs up to 'protect' them from me and mine who pass them
they have little clothing on to protect them from the sun
they shout at your dog and 'shew' them away with flapping arms and raised voice
they go from hostile to verbally abusive rapidly
their dogs are on leash in off leash areas
their dogs are off leash in on leash areas
they walk their dogs only on better weather days
their dogs are aggressive and reactive on leash, with no intervention from owner
or they like to enter the dog pound without my permission (< my brother in law)
they forget its an predator animal they have on a leash with instincts and drives, that can overtake being a human's companion at times