It's a gag where Hisoka (the dude) sees fighting as pleasure. It's problematic in the first few arcs (the 1999 anime removes a lot of the Hisoka perviness while the 2011 is more "faithful" to the manga), but it disappears after a certain point. The idea is that other characters enjoy fighting/hunting while Hisoka's whole life revolves around it and he's the driving force for the first part of the series for the main characters to grow.