(NIV1984) 1 Kings 21:21 – “‘”I am going to bring disaster on you. I will consume your descendants and cut off from Ahab every last male in Israel—slave or free.”‘”
So, yeah, Elijah is Ahab’s enemy – but not entirely. In this verse, he is proclaiming disaster on him, but he’s not the One bringing the disaster – God is.
Also, note how the curse is expanded on here. Ahab’s descendants will be consumed! And that’s a big thing in Israel. Your descendants carried on your family and legacy and, so I’ve heard, took care of you when you got too old to function. Here Ahab is basically being left completely alone – and, to some extent, his family is losing their inheritance because they won’t be around anymore to claim it. Ahab is going to undergo the same thing he just did to Naboth’s family – Naboth was probably the last of the line, since no one else in his family rose up to claim his inheritance – the vineyard.
Except, in this case, there will probably be much more bloodshed.