That's not it. The First can only appear as the dead. The First appeared as Warren, therefore, Warren was as dead as a coffin-nail. The retcon was that Warren didn't die when Willow killed him, he was teleported away by another witch, and thus, never died. He apparently was dead for a second, which is enough for Dr. First Evil.
The really bad one was where a dying Warren approached Andrew to reconcile, only to be told by a clearly confused Andrew that Warren had made him a murderer when he made him kill Jonathan. Now, sadly, this was not a story about the First making Warren his living weapon, a instrument of his only Master's dark will, enslaved by the hate that consumed him in life, oh no. It could only be that they really screwed up. The First made Andrew kill, not Warren.
I also don't get why Willow didn't fix Warren. Pottery Barn rule. He would be eternally grateful to you, you team up him and Andrew, then you get Jonathan out of Arashmaharr, you now how three of the most brilliant minds in techno-magic, all of whom owe you a favor.