If the player in question returns to the game, I think a conversation with them is the best solution. Given this is a new player, and one I presume you don’t know very well akr71, this seems to be a misunderstanding.
I would have thought the statement 'murder of helpless people is wholly evil' was self evident.
It is
literally self evident, in that it might appear to be obvious to you, but not
necessarily to others.
In a game that I am a player in, our group of players, (that are chiefly new to D&D), were in a gnome retreat searching for a mimic. Frankly, it was a bit dull going from room to room, making Wisdom: Perception and Intelligence: Investigation checks for 30 minutes of real time.
As often happens at a table, we started making out of character jokes, often at the expense of the
as written, somewhat silly gnomes. Including a non serious joke about breaking up the tedium, by attacking the gnomes.
Unfortunately one of our party mates missed the social cues that these comments were in jest, and shot an arrow at the next gnome we encountered. The group was horrified, and it was very apparent he was mystified at our reaction....he thought we were serious.
(The fact a gnome shoots a ballista bolt at the party, doesn’t help signal what is the
moral course)
I personally, believe the, aforementioned party-mate is on the autism spectrum. Moral reasoning in a context that requires imagination, varies wildly from person to person.
which to me is flat-out railroading.
Train rides can be nice

. Seriously, though,
some modeling of behavior for new players is required...a bit of hand holding can help.
Hand rails on steep stairs also restricts movement....sometimes this is warranted, so you don’t fall into the abyss.
Sigh...is this where you say that any use of a Vampiric Touch spell, is
always evil?
Typing “Facepalm” is not really an argument, so outside a vague sense of disagreement,
we don’t really know what you mean here, Flamestrike.
Chaotic Good,(in my experience), encompasses individuals whom consider their personal code of ethics to be more binding than a traditional societal code. Chaotic Good is often the alignment assigned to social reformers, in games I have played in.
A chaotic good wizard could, reasonably, have no moral qualms about using Vampiric Touch on a devil in Avernus,for example.
Neutral Good to me is the alignment of Utilitarians...concerned with promoting the maximum amount of ‘The Good’ without being overly concerned with methods...be it a traditional moral framework or code as a Lawful Good character would feel bound to, or a personal moral code as a Chaotic Good character.
As bild91 wisely pointed out, opinions vary widely on what constitutes
appropriate actions for each alignment, and frankly between editions of D&D as well.