>>48423442The premise of the show is that one of them has to kill the other, it's cruel from the getgo. You can go overboard though.
Reverse Flash and Goku Black for instance in the recent episode end up murdering tons of people and Reverse Flash blows up the sun after brutally killing Goku Black. But that makes sense, because they're both sadistic monsters that take things too far.
Mob vs Tatsumaki on the other hand has Tatsumaki launching what she perceives as a middle school student through a building for the crime of...calling her a kid. Tatsumaki's a dick, but that steps way the fuck out of line for her.
Red vs Tai has Wargreymon bloodily maiming both Red and Charizard and while they're defeated and bleeding decides to nuke them anyway, which is hilariously fucking cruel for Tai or Wargreymon. Had they just died as a result of a clash it would have felt much less jarring and weird.