A.T. Robertson Commentary Mark 15:36

"And one ran, and filling a sponge full of vinegar, put it on a reed, and gave him to drink, saying, Let be; let us see whether Elijah cometh to take him down." — Mark 15:36 (ASV)

To take him down (καθελειν αυτον). Matthew 27:49 has "to save him" (σωσων), which see for discussion.