So I was running a campaign of Out of the Abyss for DnD 5e. When I was doing my prep-work for the session I noticed that the PC's, who were level 4 at the time were currently in the town where the Demogorgon was going to be summoned by a ritual. Now upon reading this I thought that it was unfair and extremely hard to have that happen, so I thought to warn the players to run when the fight started. Unbeknownst to them, if the tried to run, i would let them disengage and escape no matter what. So when one of my players decided to attack the Demogorgon attempting to make him blind, the monster attacked back. This tentacle attack killed him in one hit and he used 2 death saves before succeeding. By this time only 3 players were left, the cleric who attempted to blind the monster, a half-elf arcane archer, a goliath monk, and a second monk. The other monk dragged the half elf archer out of the fight so the demogorgon attacked the cleric again. At this point, one of my players, the half elf started telling me that I was making the Demogorgon to hard of a fight for their low level and that I should use his other abilities, not just his attacks. He also thought I shouldn't kill the cleric because he would have to make a new character. I told him that I was playing the Demogorgon normally, attacking something that attacked him first. At this point I was told by the same player that I was a shitty DM and a sore loser. At that point I ended the session only 2 hours in and left the house we were playing at.

My question to everyone is, was I wrong to play the Demogorgon the way I did? And also should I have just let them survive no matter what and not let them die?