Todd Roybark
Hero
"What is your alignment?" The necromancer's player replies chaotic good. My wife laughed and said "I don't think so." She has been playing lawful evil as far as I'm concerned.
How new is the player to D&D? If the player thought they were extinguishing the life force of an evil thing, and replenishing their own life force so they can continue their crusade against evil...that falls under the bailiwick of Chaotic Good/ Neutral Good.
It is egotistical, but righteousness often is.
The Necromancer player clearly misstepped. Was it intentionally meant to be out of line with the ethics of the group, or unintentional? If a group of supposedly good adventures let my Chaotic Good character be murdered by a Revenant while they ate popcorn and drank wine, I would quit the group.
Having each other’s back is rule one, for adventuring groups...unless we are explicitly playing a Paranoia style game. What did your Session Zero game set as ground rules?