A couple of other reasons (besides the already mentioned one: Dutch was unarmed by the time so the Predator chose to disarm himself and to engage in melee combat):

 - Dutch was a fantastic adversary; he was clearly the strongest, the smartest, and he lasted the longest among all the preys. The delay in killing was in part to show some sense of **respect**.
 - **Dutch was also the last of the group**: the Predator was probably milking the hunt because if it killed Dutch, it wouldn't have another foe (for a while at least). In combination with the first point above: the Predator knew it wouldn't get such a great foe like Dutch again, possibly forever.