Hisoka is likely eliminating them because if he doesnt theres a chance that Chrollo could borrow some of their hatsus again making him very difficult to defeat, that was the core lesson learned from essentially dying once in their previous fight: dont fiight Chrollo when hes the one who controls all of the terms of the fight. This was also illustrated by the one scene of Chrollo ducking Hisoka until he gathered more abilities (the no way jose scene). As for Machi he obviously has a soft spot for her which may come back to bite him later on or he'll kill her near to last.
Plotwise it also serves to get Hisoka involved in the DC plot and setsup possible Kurapica team action.
I dunno, Hisoka's always been the kind of guy to want to fight someone at peak strength. Killing off the spiders only weakens his prey, and that's just very unfitting.