What does a fighter feel if he accidentally kills an opponent because of a fight?

For sure, it's a huge emotional burden on the fighter who accidentally causes the death of his opponent. Ray Mancini suffered from depression for a long time until he retired and wasn't the same fighter ever again after that fight with Duk Koo Kim. A long time ago, there was also the Griffith-Paret fight where the latter died after being knocked out.