This is also my experience with D&D combat scenes. Sometimes combat is a really bad idea, but the players are just determined to have combat, and they can't be reasoned with and no matter how many opportunities I give them to talk their way past a dangerous encounter, or de-escalate it, or even just retreat, they will still insist on combat. Then they stare at me with sad puppy-dog eyes as they watch the dice slaughter them all one by one.
Players: "Why did you put that monster in the scene if we weren't supposed to kill it?! That monster was totally OP! I demand a do-over!"
DM: "For starters, 'that monster' was King Oberon of the Fey...your warlock's patron. He was there to give you your next quest."
Players: "Well he looked menacing."
DM: "You wrote the description for your own patron. You specifically use the word 'menacing,' remember?"
Players: "Well you could have at least given us the opportunity to retreat."
DM: "I did. The guards tried to stop the fight twice but you attacked them also."
Players: "We thought the guard was trying to help the monster!"
DM: "Again--not a monster, that was your warlock patron. An important NPC."
Players: "Well you should have been more clear."
DM: "Like when I broke the fourth wall on Round One to specifically tell you that this was your warlock patron? And again on Round Two to tell you that he was immortal? And again on Round Three, to tell you that if you didn't stop you were going to die?"
Players: "...we thought you were trying to trick us."
DM: "Trick you? into not getting yourselves killed?"