Like you mate, I'm black and white through and through but I'd have certainly shown Hayden red for that challenge. Clumsy lunge from behind, studs raised, and potentially could have put his opponent out of football for weeks. Remember the LOTG are quite specific where challenges like that are concerned: Page 88
Serious foul play A tackle or challenge that endangers the safety of an opponent or uses excessive force or brutality must be sanctioned as serious foul play. Any player who lunges at an opponent in challenging for the ball from the front, from the side or from behind using one or both legs, with excessive force or endangers the safety of an opponent is guilty of serious foul play.
For me, Hayden ticks the box next to the above passage.
Kenedy should also have walked - if not for his kick out, then for playing shyte and blowing a precious 3 point result.