Matron of Malice requires 4 creatures to enter the graveyard to be resummoned at the end of turn.

My Drizzt opponent attacked with matron of Malice and 2 other creatures, procking the Guenhwyvar to also attack. I made sure to not let 4 creatures enter the graveyard. I killed the Matron of Malice and one other of his creatures. One of my creatures dies for a total of 3. Yet at end of turn Malice returned.

Only conclusion is that Guenhwyvar is counting as a creature entering the graveyard. This should not be the case, otherwise we should be able to actually kill and remove Guenhwyvar from play so it can't come back.